Summary:
When write stall has already been triggered due to number of L0 files reaching
threshold, file ingestion must proceed with its flush without waiting for the
write stall condition to cleared by the compaction because compaction can wait
for ingestion to finish (circular wait).

In order to avoid this wait, we can set `FlushOptions.allow_write_stall` to be
true (default is false). Setting it to false can cause deadlock.

This can happen when the number of compaction threads is low.

Considere the following
```
Time  compaction_thread                        ingestion_thread
 |                                             num_running_ingest_file_++
 |    while(num_running_ingest_file_>0){wait}
 |                                             flush
 V
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4751

Differential Revision: D13343037

Pulled By: riversand963

fbshipit-source-id: d3b95938814af46ec4c463feff0b50c70bd8b23f