Change the logic in KeyMayExist()

Summary:
Previously in KeyMayExist(), if DB::Get() returns non-Status::OK(), we assumes key may not exist.
However, as if index block is not in block cache, Status::Incomplete() will return. Worse still, if
options::filter_delete is enabled, we may falsely ignore the "delete" operation:

  https://github.com/facebook/rocksdb/blob/master/db/write_batch.cc#L217-L220

This diff fixes this bug and will let crash-test pass.

Test Plan:
Ran:

  ./db_stress --test_batches_snapshots=1 --ops_per_thread=1000000 --threads=32 --write_buffer_size=4194304 --destroy_db_initially=1 --reopen=0 --readpercent=5 --prefixpercent=45 --writepercent=35 --delpercent=5 --iterpercent=10 --db=/home/kailiu/local/newer --max_key=100000000 --disable_seek_compaction=0 --mmap_read=0 --block_size=16384 --cache_size=1048576 --open_files=500000 --verify_checksum=1 --sync=0 --disable_wal=0 --disable_data_sync=0 --target_file_size_base=2097152
--target_file_size_multiplier=2 --max_write_buffer_number=3 --max_background_compactions=20 --max_bytes_for_level_base=10485760 --filter_deletes=1

Previously we'll see crash happens very soon.

Reviewers: igor, dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14115

db/db_impl.cc

@@ -2698,11 +2698,17 @@ bool DBImpl::KeyMayExist(const ReadOptions& options,
                          std::string* value,
                          bool* value_found) {
   if (value_found != nullptr) {
-    *value_found = true; // falsify later if key-may-exist but can't fetch value
+    // falsify later if key-may-exist but can't fetch value
+    *value_found = true;
   }
   ReadOptions roptions = options;
   roptions.read_tier = kBlockCacheTier; // read from block cache only
-  return GetImpl(roptions, key, value, value_found).ok();
+  auto s = GetImpl(roptions, key, value, value_found);
+
+  // If options.block_cache != nullptr and the index block of the table didn't
+  // not present in block_cache, the return value will be Status::Incomplete.
+  // In this case, key may still exist in the table.
+  return s.ok() || s.IsIncomplete();
 }
 
 Iterator* DBImpl::NewIterator(const ReadOptions& options) {