[RocksDB] Added a property "leveldb.num-immutable-mem-table" so that Flush can be called without blocking, and application still has a way to check when it's done also without blocking.

Summary: as title

Test Plan: DBTest.NumImmutableMemTable

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13305

db/db_impl.cc

@@ -3039,6 +3039,9 @@ bool DBImpl::GetProperty(const Slice& property, std::string* value) {
   } else if (in == "sstables") {
     *value = versions_->current()->DebugString();
     return true;
+  } else if (in == "num-immutable-mem-table") {
+    *value = std::to_string(imm_.size());
+    return true;
   }
 
   return false;