RocksDB: enable pin_l0_filter_and_index_blocks_in_cache
RocksDB: enable pin_l0_filter_and_index_blocks_in_cache

From RocksDB documentation:

  if cache_index_and_filter_blocks is true and the below is true, then
  filter and index blocks are stored in the cache, but a reference is
  held in the "table reader" object so the blocks are pinned and only
  evicted from cache when the table reader is freed.

This is a recommended setting[1] when cache_index_and_filter_blocks is
true to minimise the performance aspect of that option, as it ensures
that L0 index & filter blocks are kept in the cache while in
use.

[1]: https://github.com/facebook/rocksdb/wiki/Memory-usage-in-RocksDB

Change-Id: Ifb879f38568188225c5c933bd51c58e8cfbe3a9e
Reviewed-on: http://review.couchbase.org/85380
Reviewed-by: Paolo Cocchi <9dd88c9f3e5cbab6e19a6c020f107e0c648ac956@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine