Add a DB property to indicate number of background errors encountered

Summary: Add a property to calculate number of background errors encountered to help users build their monitoring

Test Plan: Add a unit test. make all check

Reviewers: haobo, igor, dhruba

Reviewed By: igor

CC: ljin, nkg-, yhchiang, leveldb

Differential Revision: https://reviews.facebook.net/D16959

db/db_impl.h

@@ -109,7 +109,7 @@ class DBImpl : public DB {
                            const Slice* end);
 
   // Force current memtable contents to be flushed.
-  Status TEST_FlushMemTable();
+  Status TEST_FlushMemTable(bool wait = true);
 
   // Wait for memtable compaction
   Status TEST_WaitForFlushMemTable();