multi-kvs open fhandle and commit

updated multi-kvs test to give each thread it's own
handle to the file for performing commits

Change-Id: Ifd34f279aa5886b3b2114b05e0775eef96395f0d
MB-12991 Fix inconsistency of DB header due to concurrent WAL flush

- fdb_sync_db_header() should be invoked after grabbing
filemgr_mutex_lock().

Change-Id: Icc2f0dd62e7c700bd146186ba59ca5831e0222c5