refine timestamps in json signals api

*   API change: in update signals, allow setting a start time relative
    to now. This is an accuracy improvement in the case where the client
    has been retrying an initial request for a while. Kind of an obscure
    corner case but easy enough to address. And use a more convenient
    enum representation.

*   in update signals, choose `now` before acquiring the database lock.
    If lock acquisition takes a long time, this more accurately reflects
    the time the caller intended.

*   in general, make Time and Duration (de)serializable and use them
    in json types. This makes the types more self-describing, with
    better debug printing on both the server side and on the client
    library (in moonfire-playground). To make this work, base has to
    import serde which initially seemed like poor layering to me, but
    serde seems to be imported in some pretty foundational Rust crates
    for this reason. I'll go with it.

server/base/Cargo.toml

@@ -19,4 +19,6 @@ libc = "0.2"
 log = "0.4"
 parking_lot = { version = "0.11.1", features = [] }
 nom = "6.0.0"
+serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
 time = "0.1"

server/base/time.rs

@@ -9,6 +9,7 @@ use nom::branch::alt;
 use nom::bytes::complete::{tag, take_while_m_n};
 use nom::combinator::{map, map_res, opt};
 use nom::sequence::{preceded, tuple};
+use serde::{Deserialize, Serialize};
 use std::fmt;
 use std::ops;
 use std::str::FromStr;
@@ -19,7 +20,7 @@ type IResult<'a, I, O> = nom::IResult<I, O, nom::error::VerboseError<&'a str>>;
 pub const TIME_UNITS_PER_SEC: i64 = 90_000;
 
 /// A time specified as 90,000ths of a second since 1970-01-01 00:00:00 UTC.
-#[derive(Clone, Copy, Default, Eq, Ord, PartialEq, PartialOrd)]
+#[derive(Clone, Copy, Default, Deserialize, Eq, Ord, PartialEq, PartialOrd, Serialize)]
 pub struct Time(pub i64);
 
 /// Returns a parser for a `len`-digit non-negative number which fits into an i32.
@@ -221,7 +222,7 @@ impl fmt::Display for Time {
 /// A duration specified in 1/90,000ths of a second.
 /// Durations are typically non-negative, but a `moonfire_db::db::CameraDayValue::duration` may be
 /// negative.
-#[derive(Clone, Copy, Debug, Default, Eq, Ord, PartialEq, PartialOrd)]
+#[derive(Clone, Copy, Default, Deserialize, Eq, Ord, PartialEq, PartialOrd, Serialize)]
 pub struct Duration(pub i64);
 
 impl Duration {
@@ -230,6 +231,13 @@ impl Duration {
     }
 }
 
+impl fmt::Debug for Duration {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        // Write both the raw and display forms.
+        write!(f, "{} /* {} */", self.0, self)
+    }
+}
+
 impl fmt::Display for Duration {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let mut seconds = self.0 / TIME_UNITS_PER_SEC;