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

db/table_cache.h

@@ -48,9 +48,9 @@ class TableCache {
              const Slice& k,
              void* arg,
              bool (*handle_result)(void*, const Slice&, const Slice&, bool),
-             bool* tableIO,
+             bool* table_io,
              void (*mark_key_may_exist)(void*) = nullptr,
-             const bool no_IO = false);
+             const bool no_io = false);
 
   // Evict any entry for the specified file number
   void Evict(uint64_t file_number);
@@ -62,9 +62,9 @@ class TableCache {
   const EnvOptions& storage_options_;
   std::shared_ptr<Cache> cache_;
 
-  Status FindTable(const EnvOptions& toptions,
-                   uint64_t file_number, uint64_t file_size, Cache::Handle**,
-                   bool* tableIO = nullptr);
+  Status FindTable(const EnvOptions& toptions, uint64_t file_number,
+                   uint64_t file_size, Cache::Handle**, bool* table_io=nullptr,
+                   const bool no_io = false);
 };
 
 }  // namespace leveldb