This reduces the working set by another 960 bytes for a typical one-hour recording, improving cache efficiency a bit more.
8 bytes from SampleIndexIterator:
* reduce the three "bytes" fields to two. Doing so as "bytes_key" vs
"bytes_nonkey" slowed it down a bit, perhaps because the "bytes" is
needed right away and requires a branch. But "bytes" vs "bytes_other"
seems fine. Looks like it can do this with cmovs in parallel with other
stuff.
* stuff "is_key" into the "i" field.
8 bytes from recording::Segment itself:
* make "frames" and "key_frame" u16s
* stuff "trailing_zero" into "video_sample_entry_id"
This makes it easier to understand which options are valid with each
command.
Additionally, there's more separation of implementations. The most
obvious consequence is that "moonfire-nvr ts ..." no longer uselessly
locks/opens a database.
