Fixing unreleased column_family ref when using options.ini for rocksdb

[littlepig2013]

When specifying 'options.ini' to benchmark rocksdb, there would be an assertion error because the column family refs are not fully released yet. For example, /ycsb -load -run -db rocksdb -P workloads/workloadb -p rocksdb.optionsfile=rocksdb/options.ini ends up with the following assertion error:

db/column_family.cc:1618: rocksdb::ColumnFamilySet::~ColumnFamilySet(): Assertion `last_ref' failed.

This commit makes the cf_handles a private member of the rocksdb instance and releases the refs by deleting the pointers stored in cf_handles.

[ls4154]

Hi, thank you for finding the bug. I missed it since I don't use option files often.

One problem is that RocksdbDB::Cleanup will be called by each client thread.
So we need to prevent race for handle deletion.

I think deleting handles should be done after the lock_guard and refcount check.
Can yo add a commit to fix this?

[ls4154]

In addition, since rocksdb instance is shared across threads, cf_handle should be a static variable.
The thread that opens db and the thread that deletes db might be different.

[littlepig2013]

Thanks for your feedback! I added a new comment with fixing your comments!

[ls4154]

Merged into master. I've added a missing static cf_handles_ definition.

Thanks for your contribution!