Fetch snapshot by time

[alecgibson]

This change adds the ability fetch a snapshot by time. The motivation
for this is that fetching a document by time is quite a "natural" way
to think about document history, and allows us to - for example - fetch
multiple documents as they were at a given time, without having to
look up their exact version numbers first.

We add a new Connection.fetchSnapshotByTimestamp method, which
follows a very similar route to Connection.fetchSnapshot, and where
possible, as much code is re-used as possible:

• both methods use a subclassed child of SnapshotRequest
• both methods have their requests handled by the same machinery in
  Connection
• both methods in the Backend have ops applied by a common method,
  but use their own methods for calls to middleware

In order to make this feature possible at scale, this change also adds
two new methods to the MilestoneDB interface:

• getMilestoneSnapshotAtOrBeforeTime
• getMilestoneSnapshotAtOrAfterTime

These methods are used to fetch milestone snapshots either side of the
requested timestamp, which means we only need to fetch the ops between
the two of them to reach the desired timestamp.

In the case where a milestone database is not being used, then fetching
a snapshot by timestamp is still possible, but it will fetch all the ops
for a document, and keep applying them from v0 until the timestamp is
reached, which is not particularly scalable.

[coveralls]

Coverage increased (+0.2%) to 95.848% when pulling 8e46271 on snapshot-by-timestamp into 95c81b8 on master.

[ericyhwang]

Here's the notes I took during the PR review meeting today.

I haven't looked at the tests yet, but the actual code itself looks great. Glad you could find a clean abstraction boundary.

[alecgibson]

@nateps / @ericyhwang as discussed, I've moved the snapshot building function into ot.js.

[ericyhwang]

LGTM from me and Nate