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/db_impl.h

@@ -103,6 +103,15 @@ class DBImpl : public DB {
   // Trigger's a background call for testing.
   void TEST_PurgeObsoleteteWAL();
 
+  // KeyMayExist's internal function, but can be called internally from rocksdb
+  // to check memtable from sequence_number=read_from_seq. This is useful to
+  // check presence of key in db when key's existence is to be also checked in
+  // an incompletely written WriteBatch in memtable. eg. Database doesn't have
+  // key A and WriteBatch=[PutA,B; DelA]. A KeyMayExist called from DelA also
+  // needs to check itself for any PutA to be sure to not drop the delete.
+  bool KeyMayExistImpl(const Slice& key,
+                       const SequenceNumber read_from_seq);
+
  protected:
   Env* const env_;
   const std::string dbname_;
@@ -399,11 +408,11 @@ class DBImpl : public DB {
     std::vector<SequenceNumber>& snapshots,
     SequenceNumber* prev_snapshot);
 
-  // Function that Get and KeyMayExist call with no_IO true or false
+  // Function that Get and KeyMayExistImpl call with no_io true or false
   Status GetImpl(const ReadOptions& options,
                  const Slice& key,
                  std::string* value,
-                 const bool no_IO = false);
+                 const bool no_io = false);
 };
 
 // Sanitize db options.  The caller should delete result.info_log if