Fixes a problem with how we count num_active_writes in the merger serializer.
Remove ul suffix from MESSAGE_SCHEDULER_GRANULARITY

Otherwise it does not compile on 32 bit platforms.

Review 998 by Daniel Mewes
fixing python rql tests
Merge branch 'daniel_1471' into next
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next
Merge branch 'next' into daniel_backfill_latency_tweaks