Fix file opening bug in multi-threaded test
Fix file opening bug in multi-threaded test

- On super fast storage such as RAM disk, DB file compaction can be
executed before other threads even open the file. This causes problem
because the worker threads make the DB file name using the compaction
number. This is not a bug in ForestDB, but in the test itself.
- Now all threads wait until other threads open the DB file.

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