Ability for rocksdb to compact when flushing the in-memory memtable to a file in L0.

Summary:
Rocks accumulates recent writes and deletes in the in-memory memtable.
When the memtable is full, it writes the contents on the memtable to
a file in L0.

This patch removes redundant records at the time of the flush. If there
are multiple versions of the same key in the memtable, then only the
most recent one is dumped into the output file. The purging of
redundant records occur only if the most recent snapshot is earlier
than the earliest record in the memtable.

Should we switch on this feature by default or should we keep this feature
turned off in the default settings?

Test Plan: Added test case to db_test.cc

Reviewers: sheki, vamsi, emayanke, heyongqiang

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8991

include/leveldb/options.h

@@ -380,6 +380,10 @@ struct Options {
   // as well as prevent overallocation for mounts that preallocate
   // large amounts of data (such as xfs's allocsize option).
   size_t manifest_preallocation_size;
+
+  // Purge duplicate/deleted keys when a memtable is flushed to storage.
+  // Default: true
+  bool purge_redundant_kvs_while_flush;
 };
 
 // Options that control read operations