Use KeyMayExist for WriteBatch-Deletes

Summary:
Introduced KeyMayExist checking during writebatch-delete and removed from Outer Delete API because it uses writebatch-delete.
Added code to skip getting Table from disk if not already present in table_cache.
Some renaming of variables.
Introduced KeyMayExistImpl which allows checking since specified sequence number in GetImpl useful to check partially written writebatch.
Changed KeyMayExist to not be pure virtual and provided a default implementation.
Expanded unit-tests in db_test to check appropriately.
Ran db_stress for 1 hour with ./db_stress --max_key=100000 --ops_per_thread=10000000 --delpercent=50 --filter_deletes=1 --statistics=1.

Test Plan: db_stress;make check

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb, xjin

Differential Revision: https://reviews.facebook.net/D11745

db/write_batch_internal.h

@@ -7,6 +7,8 @@
 
 #include "leveldb/types.h"
 #include "leveldb/write_batch.h"
+#include "leveldb/db.h"
+#include "leveldb/options.h"
 
 namespace leveldb {
 
@@ -39,7 +41,12 @@ class WriteBatchInternal {
 
   static void SetContents(WriteBatch* batch, const Slice& contents);
 
-  static Status InsertInto(const WriteBatch* batch, MemTable* memtable);
+  // Inserts batch entries into memtable
+  // Drops deletes in batch if filter_del is set to true and
+  // db->KeyMayExist returns false
+  static Status InsertInto(const WriteBatch* batch, MemTable* memtable,
+                           const Options* opts = nullptr, DB* db = nullptr,
+                           const bool filter_del = false);
 
   static void Append(WriteBatch* dst, const WriteBatch* src);
 };