SERVER-15986 added storage_engine_metadata - reads and writes storage engine metadata file in data directory

(cherry picked from commit a932b12ac03366e9336ffc92e7710ddb5c3b3a39)
SERVER-15986 SERVER-16283 improve validation to detect mmapv1 local.ns files

(cherry picked from commit bb8e5e6b28f31a0b4e9eefc5df52581073bf332d)