RocksDBState tests fail on OSX

[Eric Weaver]

When running tests on OSX RocksDBState tests fail with the exception below. This appears to be an issue with some RocksDB ColumnFamily pointers still left open prior to destroying the DB.

Assertion failed: (refs_.load(std::memory_order_relaxed) == 0), function ~ColumnFamilyData, file db/column_family.cc, line 443.

Process finished with exit code 134 (interrupted by signal 6: SIGABRT)

Reproducing steps:

mvn -Dtest=RocksDBStateTest test

OS info:

 System Version: macOS 10.12.6 (16G1710)
 Kernel Version: Darwin 16.7.0