SERVER-15997 Make sure stepdown can be interrupted with killop
SERVER-15986 added storage_engine_metadata - reads and writes storage engine metadata file in data directory
SERVER-15986 prevent server from starting up when non-active storage engine files are detected
             added storage_engine_metadata to mongo/db/storage/SConscript
             fixed cleanbb.py so that cleanup() will remove directories as well.
             fixed unix_socket1.js so that it does not store socket file in data directory
Merge remote-tracking branch 'upstream/master'