Update rocksdb to v5.18.3

CHANGELOG.md

 # Changelog
 All notable changes to this project will be documented in this file.
 
-### Unreleased
+## Unreleased
 - Better memory management and recycling, putting less pressure on the global
 memory allocator.
 - Addition of pub/sub support, commands implemented: ``PUBLISH``, ``SUBSCRIBE``,
   ``PSUBSCRIBE``, ``UNSUBSCRIBE``, ``PUNSUBSCRIBE``
 - Addition of ``--steal-state-machine`` flag to ``quarkdb-create`` to make
   transition from bulkload mode to raft easier.
+- Updated rocksdb to v5.18.3.
 
 ## 0.3.5 (2018-11-28)
 - Updated rocksdb dependency to v5.17.2.

rocksdb @ 641fae60

-Subproject commit f438b98e552010613cff8684a7d84191ed785148
+Subproject commit 641fae60f63619ed5d0c9d9e4c4ea5a0ffa3e253