Summary:
RocksDB should decrement the counter `unscheduled_flushes_` as soon as the bg
thread is scheduled. Before this fix, the counter is decremented only when the
bg thread starts and picks an element from the flush queue. This may cause more
than necessary bg threads to be scheduled. Not a correctness issue, but may
affect flush thread count.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6104
Test Plan:
```
make check
```
Differential Revision: D18735584
Pulled By: riversand963
fbshipit-source-id: d36272d4a08a494aeeab6200a3cff7a3d1a2dc10
09fcf4fb6b