assert fail

https://github.com/facebook/rocksdb/blob/d77476ef557078d89f545efbe33bc8825b07237a/db/db_impl_compaction_flush.cc#L142

when rocksdb do flush job, it holds all column family which need to be flushed, and then flush column family one by one.

consider below scenario:
1. flush thread 1 pick column family a、b、c, then flush a (release the mutex lock).
2. user write column family c and memtable full, column family c will be inserted into flush queue.
3. flush thread 2 pick column family c, then flush c (pick all available memtables)
4. flush thread 1 done column family a、b
5. flush thread 2 done column c
6. flush thread begin to do column family c, no available memtable to flush, assert fail