rocksdb: Updated to v6.1.2

Signed-off-by: Mark Nelson <mnelson@redhat.com>

diff --git a/src/rocksdb b/src/rocksdb
index 37828c548a886dccf58a7a93fc2ce13877884c0c..4c736f177851cbf9fb7a6790282306ffac5065f8 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit 37828c548a886dccf58a7a93fc2ce13877884c0c
+Subproject commit 4c736f177851cbf9fb7a6790282306ffac5065f8