Avoid reloading filter on Get() if cache_index_and_filter_blocks == false

Summary:
This fixes the case that filter policy is missing in SST file, but we
open the table with filter policy on and cache_index_and_filter_blocks =
false. The current behavior is that we will try to load it every time on
Get() but fail.

Test Plan: unit test

Reviewers: yhchiang, igor, rven, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D25455