use Blake3 instead of SHA-1 or Blake2b

Benefits:

* Blake3 is faster. This is most noticeable for the hashing of the
  sample file data.
* we no longer need OpenSSL, which helps with shrinking the binary size
  (#70). sha1 basically forced OpenSSL usage; ring deliberately doesn't
  support this old algorithm, and the pure-Rust sha1 crate is painfully
  slow. OpenSSL might still be a better choice than ring/rustls for TLS
  but it's nice to have the option.

For the video sample entries, I decided we don't need to hash at all. I
think the id number is sufficiently stable, and it's okay---perhaps even
desirable---if an existing init segment changes for fixes like e5b83c2.

design/api.md

@@ -290,16 +290,13 @@ arbitrary order. Each recording object has the following properties:
     greater than the requested `endTime90k` if this recording was ongoing at
     the requested time.
 *   `videoSampleEntryId`: a reference to an entry in the `videoSampleEntries`
-    map. These ids are strings so that they can serve as JSON object keys.
+    map.mp4` URL.
 *   `videoSamples`: the number of samples (aka frames) of video in this
     recording.
 
 Under the property `videoSampleEntries`, an object mapping ids to objects with
 the following properties:
 
-*   `sha1`: a SHA-1 hash of the ISO/IEC 14496-12 section 8.5.2
-    `VisualSampleEntry` bytes. The actual bytes can be retrieved, wrapped into
-    an initialization segment `.mp4`, at the URL `/api/init/<sha1>.mp4`.
 *   `width`: the stored width in pixels.
 *   `height`: the stored height in pixels.
 *   `pixelHSpacing`: the relative width of a pixel, as in a ISO/IEC 14496-12
@@ -307,6 +304,9 @@ the following properties:
 *   `pixelVSpacing`: the relative height of a pixel, as in a ISO/IEC 14496-12
     section 12.1.4.3 `PixelAspectRatioBox`. If absent, assumed to be 1.
 
+The full initialization segment data for a given video sample entry can be
+retrieved at the URL `/api/init/<id>.mp4`.
+
 Example request URI (with added whitespace between parameters):
 
 ```
@@ -335,7 +335,6 @@ Example response:
   ],
   "videoSampleEntries": {
     "1": {
-      "sha1": "81710c9c51a02cc95439caa8dd3bc12b77ffe767",
       "width": 1280,
       "height": 720
     }
@@ -468,7 +467,7 @@ Content-Type: video/mp4; codecs="avc1.640028"
 X-Recording-Id: 42.5680
 X-Recording-Start: 130985461191810
 X-Time-Range: 5220058-5400061
-X-Video-Sample-Entry-Sha1: 25fad1b92c344dadc0473a783dff957b0d7d56bb
+X-Video-Sample-Entry-Id: 4
 
 binary mp4 data
 ```
@@ -478,7 +477,7 @@ Content-Type: video/mp4; codecs="avc1.640028"
 X-Recording-Id: 42.5681
 X-Recording-Start: 130985461191822
 X-Time-Range: 0-180002
-X-Video-Sample-Entry-Sha1: 25fad1b92c344dadc0473a783dff957b0d7d56bb
+X-Video-Sample-Entry-Id: 4
 
 binary mp4 data
 ```
@@ -488,7 +487,7 @@ Content-Type: video/mp4; codecs="avc1.640028"
 X-Recording-Id: 42.5681
 X-Recording-Start: 130985461191822
 X-Time-Range: 180002-360004
-X-Video-Sample-Entry-Sha1: 25fad1b92c344dadc0473a783dff957b0d7d56bb
+X-Video-Sample-Entry-Id: 4
 
 binary mp4 data
 ```
@@ -507,13 +506,13 @@ active HTTP/1.1 connections: six in Chrome's case. The WebSocket limit is much
 higher (256), allowing browser-side Javascript to stream all active camera
 streams simultaneously as well as making other simultaneous HTTP requests.
 
-### `GET /api/init/<sha1>.mp4`
+### `GET /api/init/<id>.mp4`
 
 Returns a `.mp4` suitable for use as a [HTML5 Media Source Extensions
 initialization segment][init-segment]. The MIME type will be `video/mp4`, with
 a `codecs` parameter as specified in [RFC 6381][rfc-6381].
 
-### `GET /api/init/<sha1>.mp4.txt`
+### `GET /api/init/<id>.mp4.txt`
 
 Returns a `text/plain` debugging string for the `.mp4` generated by the
 same URL minus the `.txt` suffix.

design/schema.md

@@ -522,7 +522,7 @@ The snippet below is a illustrative excerpt of the SQLite schema; see
       camera_id integer references camera (id) not null,
 
       sample_file_uuid blob unique not null,
-      sample_file_sha1 blob,
+      sample_file_blake3 blob,
       sample_file_size integer,
 
       -- The starting time and duration of the recording, in 90 kHz units since
@@ -540,11 +540,10 @@ The snippet below is a illustrative excerpt of the SQLite schema; see
     -- A concrete box derived from a ISO/IEC 14496-12 section 8.5.2
     -- VisualSampleEntry box. Describes the codec, width, height, etc.
     create table visual_sample_entry (
-      -- A SHA-1 hash of |bytes|.
-      sha1 blob primary key,
+      id integerprimary key,
 
       -- The width and height in pixels; must match values within
-      |sample_entry_bytes|.
+      -- `sample_entry_bytes`.
       width integer,
       height integer,