Data block GC now runs in a coroutine, and concurrency is scaled based on the garbage ratio.
Removed an obsolete TODO
Merge branch 'next' into daniel_concurrent_gc
Simplified termination of GC coros
More reasonable GC range
Increased extent size to 2 MB. That improves GC behavior so much.
Increased extent size to 2 MB. Works much better with GC.
Merge branch 'daniel_concurrent_gc' of github.com:rethinkdb/rethinkdb into daniel_concurrent_gc

Conflicts:
	src/config/args.hpp
Merge branch 'next' into daniel_concurrent_gc
Fix for #2500
Merge branch 'daniel_2500' into daniel_concurrent_gc

Conflicts:
	src/arch/io/disk/conflict_resolving.cc
Merge branch 'daniel_concurrent_gc' of github.com:rethinkdb/rethinkdb into daniel_concurrent_gc
Merge branch 'next' into daniel_concurrent_gc
Address issues raised in CR.
Merge branch 'next' into daniel_concurrent_gc