refactor TableCache Get/NewIterator for single exit points
refactor TableCache Get/NewIterator for single exit points

Summary:
these functions were too complicated to change with exit points everywhere, so refactored them.

btw, please review urgently, this is a prereq to fix the 5.0 perf regression
Closes https://github.com/facebook/rocksdb/pull/1534

Differential Revision: D4198972

Pulled By: ajkr

fbshipit-source-id: 04ebfb7

SunguckLee/RocksDB,wat-ze-hex/rocksdb,vmx/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,vmx/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,SunguckLee/RocksDB