Summary:
This patch changes concurrency guarantees around ColumnFamilySet::column_families_ and ColumnFamilySet::column_families_data_.

Before:
* When mutating: lock DB mutex and spin lock
* When reading: lock DB mutex OR spin lock

After:
* When mutating: lock DB mutex and be in write thread
* When reading: lock DB mutex or be in write thread

That way, we eliminate the spin lock that protects these hash maps and  simplify concurrency. That means we don't need to lock the spin lock during writing, since writing is mutually exclusive with column family create/drop (the only operations that mutate those hash maps).

With these new restrictions, I also needed to move column family create to the write thread (column family drop was already in the write thread).

Even though we don't need to lock the spin lock during write, impact on performance should be minimal -- the spin lock is almost never busy, so locking it is almost free.

This addresses task t5116919.

Test Plan:
make check

Stress test with lots and lots of column family drop and create:

   time ./db_stress --threads=30 --ops_per_thread=5000000 --max_key=5000 --column_families=200 --clear_column_family_one_in=100000 --verify_before_write=0  --reopen=15 --max_background_compactions=10 --max_background_flushes=10 --db=/fast-rocksdb-tmp/db_stress/

Reviewers: yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D30651