SERVER-15580 KeepMutationsStage shouldn't deref invalidated iterator
CollectionInfoCache::reset should reset index keys under X lock
Revert "SERVER-15831 top chunk auto split tests"

This reverts commit ce6a6a4f55c29286860ac1993d5929da0205ff8c.
SERVER-14425: handle deadlocks in the middle up a multi-update
SERVER-15623 Test that w:0 is prohibited in getLastErrorDefaults.

Closes #847

Signed-off-by: Benety Goh <benety@mongodb.com>