Update ForestDB wrapper
Update ForestDB wrapper

- Added prefetching duration config.

- FDB_RESULT_FAIL is not used anymore.

couchbaselabs/ForestDB-Benchmark,couchbaselabs/ForestDB-Benchmark,yyangpan/ForestDB-Benchmark,yyangpan/ForestDB-Benchmark,yyangpan/ForestDB-Benchmark