Use mutex instead of spinlock for synchronizing a file manager.
Use mutex instead of spinlock for synchronizing a file manager.

We should avoid using a spinlock if a critical section can have
a long running time.

Change-Id: I0ce61a719be370694b9a464341efe84e351c4add

hisundar/forestdb,nvoron23/forestdb,seem-sky/forestdb,couchbase/forestdb,joliss/forestdb,yyangpan/forestdb,seem-sky/forestdb,jinxuan/forestdb,hisundar/forestdb,jinxuan/forestdb,zhaog/forestdb,sushmaad/forestdb,wurikiji/forestdb,zhaog/forestdb,nvoron23/forestdb,sushmaad/forestdb,yyangpan/forestdb,wurikiji/forestdb,couchbase/forestdb,joliss/forestdb