Add endpoint to get arbitrary match sequences

This patch adds an endpoint at /matches/<arena> that takes an argument
in its query string called "numbers".  "numbers" is a comma-separated
list of match numbers that the requester would like information about.

This endpoint also supports three special values that can be used in
place of match numbers:

 - "current": for the current match
 - "next": for the next match
 - "next+1": for the next-but-one match

Note that there may be no current match, and there may be no next
match.  So those items may resolve to null.

For example: /matches/A?numbers=current,next

May return { matches = [ null, { ... match info ... } ] } if there is
no current match, but there is on in the future.

(Maybe in the future "next+$N" will be supported, but for now it's
not. We have an application that needs next+1 now.)
Bump srcomp submodule

To get match_after() function
Expose the number of the latest scored match.

This is useful to give the teams an idea of what scores have been
input, so that they can know when they change.