[Performance Branch] If options.max_open_files set to be -1, cache table readers in FileMetadata for Get() and NewIterator()

Summary:
In some use cases, table readers for all live files should always be cached. In that case, there will be an opportunity to avoid the table cache look-up while Get() and NewIterator().

We define options.max_open_files = -1 to be the mode that table readers for live files will always be kept. In that mode, table readers are cached in FileMetaData (with a reference count hold in table cache). So that when executing table_cache.Get() and table_cache.newInterator(), LRU cache checking can be by-passed, to reduce latency.

Test Plan: add a test case in db_test

Reviewers: haobo, kailiu

Reviewed By: haobo

CC: dhruba, igor, leveldb

Differential Revision: https://reviews.facebook.net/D15039