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

table/table.cc

@@ -322,7 +322,9 @@ Iterator* Table::NewIterator(const ReadOptions& options) const {
 Status Table::InternalGet(const ReadOptions& options, const Slice& k,
                           void* arg,
                           bool (*saver)(void*, const Slice&, const Slice&,
-                                        bool)) {
+                                        bool),
+                          void (*mark_key_may_exist)(void*),
+                          const bool no_IO) {
   Status s;
   Iterator* iiter = rep_->index_block->NewIterator(rep_->options.comparator);
   bool done = false;
@@ -338,6 +340,11 @@ Status Table::InternalGet(const ReadOptions& options, const Slice& k,
       // cross one data block, we should be fine.
       RecordTick(rep_->options.statistics, BLOOM_FILTER_USEFUL);
       break;
+    } else if (no_IO) {
+        // Update Saver.state to Found because we are only looking for whether
+        // bloom-filter can guarantee the key is not there when "no_IO"
+        (*mark_key_may_exist)(arg);
+        done = true;
     } else {
       bool didIO = false;
       Iterator* block_iter = BlockReader(this, options, iiter->value(),

table/table.h

@@ -86,7 +86,9 @@ class Table {
   Status InternalGet(
       const ReadOptions&, const Slice& key,
       void* arg,
-      bool (*handle_result)(void* arg, const Slice& k, const Slice& v, bool));
+      bool (*handle_result)(void* arg, const Slice& k, const Slice& v, bool),
+      void (*mark_key_may_exist)(void*) = nullptr,
+      const bool no_IO = false);
 
 
   void ReadMeta(const Footer& footer);