Fix buffer cache race condition under concurrent file open
Fix buffer cache race condition under concurrent file open

- When multiple threads try to open same file concurrently,
initializing the buffer cache structures to be dedicated to the
file fails due to race condition.

Change-Id: I5ac525980bff3097ab4f73fa7201d7f81046a468

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