MariaRocks produces numerous (spurious?) valgrind failures

Any test with MariaRocks produces a lot of valgrind failures.

• A lot are probably harmless ("still reachable" failures for objects that are allocated once and then never freed)
• However we should have suppressions for them
• There are also "blocks are indirectly lost" and "blocks are definitely lost" failures. These are do not look harmless, and they come from upstream: https://github.com/facebook/mysql-5.6/issues/586 .