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

include/leveldb/db.h

@@ -122,8 +122,12 @@ class DB {
 
   // If the key definitely does not exist in the database, then this method
   // returns false. Otherwise return true. This check is potentially
-  // lighter-weight than invoking DB::Get(). No IO is performed
-  virtual bool KeyMayExist(const Slice& key) = 0;
+  // lighter-weight than invoking DB::Get(). One way to make this lighter weight
+  // is to avoid doing any IOs
+  // Default implementation here returns true
+  virtual bool KeyMayExist(const Slice& key) {
+    return true;
+  }
 
   // Return a heap-allocated iterator over the contents of the database.
   // The result of NewIterator() is initially invalid (caller must

include/leveldb/options.h

@@ -472,7 +472,7 @@ struct Options {
   // only incurs in-memory look up. This optimization avoids writing the delete
   // to storage when appropriate.
   // Default: false
-  bool deletes_check_filter_first;
+  bool filter_deletes;
 
 };