post 2.8.0-rc3
SERVER-16432 Chunk migration data structures not properly synchronized

Fix receipient side.
SERVER-14306 Make sure mongos never requests more results than needed from the shards
SERVER-16431 Move the acquisition of the flush lock to a separate method

Also adds assertions and new tests. No functional changes yet - this is in
preparation for removal of the parallel batch writer lock.
Merge remote-tracking branch 'upstream/master'