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

db/table_cache.cc

@@ -112,14 +112,16 @@ Status TableCache::Get(const ReadOptions& options,
                        const Slice& k,
                        void* arg,
                        bool (*saver)(void*, const Slice&, const Slice&, bool),
-                       bool* tableIO) {
+                       bool* tableIO,
+                       void (*mark_key_may_exist)(void*),
+                       const bool no_IO) {
   Cache::Handle* handle = nullptr;
   Status s = FindTable(storage_options_, file_number, file_size,
                        &handle, tableIO);
   if (s.ok()) {
     Table* t =
       reinterpret_cast<Table*>(cache_->Value(handle));
-    s = t->InternalGet(options, k, arg, saver);
+    s = t->InternalGet(options, k, arg, saver, mark_key_may_exist, no_IO);
     cache_->Release(handle);
   }
   return s;