Compact multiple memtables before flushing to storage.

Summary:
Merge multiple multiple memtables in memory before writing it
out to a file in L0.

There is a new config parameter min_write_buffer_number_to_merge
that specifies the number of write buffers that should be merged
together to a single file in storage. The system will not flush
wrte buffers to storage unless at least these many buffers have
accumulated in memory.
The default value of this new parameter is 1, which means that
a write buffer will be immediately flushed to disk as soon it is
ready.

Test Plan: make check

Differential Revision: https://reviews.facebook.net/D11241

db/db_impl.h

@@ -152,7 +152,7 @@ class DBImpl : public DB {
   // for the entire period. The second method WriteLevel0Table supports
   // concurrent flush memtables to storage.
   Status WriteLevel0TableForRecovery(MemTable* mem, VersionEdit* edit);
-  Status WriteLevel0Table(MemTable* mem, VersionEdit* edit,
+  Status WriteLevel0Table(std::vector<MemTable*> &mems, VersionEdit* edit,
                                 uint64_t* filenumber);
 
   Status MakeRoomForWrite(bool force /* compact even if there is room? */);