Returning an unlimited number of elements from any endpoint is a bad
idea. For the notes endpoint specifically, we return all notes since
a given `X-Last-Fetched-At` value. The backend specifies the new value
to use for that item in the response, so if we return a limited set of
notes and use an appropriate value, we get a rudimentary form of keyset
pagination for free.

The way the notes endpoint is used today, this is mostly a theoretical
worry, and in practice, basically every response will fit within a
single page anyway. However, we can use the `notes` endpoint *instead
of* the `discussions` endpoint to fill a merge request discussion from
the beginning of time. When we do this, large discussions are loaded in
batches automatically, using pre-existing frontend code. So this change
is best seen as preparatory work for removing `discussions` altogether.

This commit also fixes a pre-existing problem with the endpoint, which
is the fact that `updated_at` is stored with *microsecond* precision in
the database. Converting the X-Last-Fetched-At value from integer to
float allows us to avoid a range of boundary conditions that result.