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/memtablelist.h

@@ -18,7 +18,7 @@ class Mutex;
 class MemTableListIterator;
 
 //
-// This class stores refeernces to all the immutable memtables.
+// This class stores references to all the immutable memtables.
 // The memtables are flushed to L0 as soon as possible and in
 // any order. If there are more than one immutable memtable, their
 // flushes can occur concurrently.  However, they are 'committed'
@@ -49,14 +49,13 @@ class MemTableList {
 
   // Returns true if there is at least one memtable on which flush has
   // not yet started.
-  bool IsFlushPending();
+  bool IsFlushPending(int min_write_buffer_number_to_merge);
 
-  // Returns the earliest memtable that needs to be flushed.
-  // Returns null, if no such memtable exist.
-  MemTable* PickMemtableToFlush();
+  // Returns the earliest memtables that needs to be flushed.
+  void PickMemtablesToFlush(std::vector<MemTable*>* mems);
 
   // Commit a successful flush in the manifest file
-  Status InstallMemtableFlushResults(MemTable* m,
+  Status InstallMemtableFlushResults(const std::vector<MemTable*> &m,
                       VersionSet* vset, Status flushStatus,
                       port::Mutex* mu, Logger* info_log,
                       uint64_t file_number,