Make rocksdb-deletes faster using bloom filter

Summary:
Wrote a new function in db_impl.c-CheckKeyMayExist that calls Get but with a new parameter turned on which makes Get return false only if bloom filters can guarantee that key is not in database. Delete calls this function and if the option- deletes_use_filter is turned on and CheckKeyMayExist returns false, the delete will be dropped saving:
1. Put of delete type
2. Space in the db,and
3. Compaction time

Test Plan:
make all check;
will run db_stress and db_bench and enhance unit-test once the basic design gets approved

Reviewers: dhruba, haobo, vamsi

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11607

include/leveldb/db.h

@@ -120,6 +120,11 @@ class DB {
                                        const std::vector<Slice>& keys,
                                        std::vector<std::string>* values) = 0;
 
+  // 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;
+
   // Return a heap-allocated iterator over the contents of the database.
   // The result of NewIterator() is initially invalid (caller must
   // call one of the Seek methods on the iterator before using it).

include/leveldb/options.h

@@ -465,6 +465,15 @@ struct Options {
   // Default: 0
   uint64_t bytes_per_sync;
 
+  // Use bloom-filter for deletes when this is true.
+  // db->Delete first calls KeyMayExist which checks memtable,immutable-memtable
+  // and bloom-filters to determine if the key does not exist in the database.
+  // If the key definitely does not exist, then the delete is a noop.KeyMayExist
+  // only incurs in-memory look up. This optimization avoids writing the delete
+  // to storage when appropriate.
+  // Default: false
+  bool deletes_check_filter_first;
+
 };
 
 // Options that control read operations

include/leveldb/statistics.h

@@ -58,7 +58,9 @@ enum Tickers {
   NUMBER_MULTIGET_KEYS_READ = 19,
   NUMBER_MULTIGET_BYTES_READ = 20,
 
-  TICKER_ENUM_MAX = 21
+  NUMBER_FILTERED_DELETES = 21,
+
+  TICKER_ENUM_MAX = 22
 };
 
 const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
@@ -82,7 +84,8 @@ const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
   { NO_ITERATORS, "rocksdb.num.iterators" },
   { NUMBER_MULTIGET_CALLS, "rocksdb.number.multiget.get" },
   { NUMBER_MULTIGET_KEYS_READ, "rocksdb.number.multiget.keys.read" },
-  { NUMBER_MULTIGET_BYTES_READ, "rocksdb.number.multiget.bytes.read" }
+  { NUMBER_MULTIGET_BYTES_READ, "rocksdb.number.multiget.bytes.read" },
+  { NUMBER_FILTERED_DELETES, "rocksdb.number.deletes.filtered" }
 };
 
 /**