Make the first writer only performs compaction in multi-thread test
Make the first writer only performs compaction in multi-thread test

- When more than one thread tries to call fdb_compact() concurrently,
only one thread can perform compaction while the other callings are
denied. In multi-thread test, this can cause miscounting of filename
counter so that the test may sporadically fail (it is a false-positive
alarm). To avoid this, only one thread performs compaction, thus there
is no race on calling fdb_compact() anymore.

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