Calling DB::open() with create_missing_column_families=true is quadratic

[sadderchris]

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Expected behavior

Per the notes here, when calling DB::open() with create_missing_column_families=true, column family creation is batched and takes a reasonable amount of time when opening a fresh database with lots of new column families.

Actual behavior

Calling DB::open() with create_missing_column_families=true doesn't actually batch create new column families and instead is quadratic in the number of column families it needs to create.

rocksdb/db/db_impl/db_impl_open.cc

Lines 2056 to 2060 in 8b56696

	// missing column family, create it
	ColumnFamilyHandle* handle = nullptr;
	impl->mutex_.Unlock();
	s = impl->CreateColumnFamily(cf.options, cf.name, &handle);
	impl->mutex_.Lock();

Steps to reproduce the behavior

1. Set options.create_missing_column_families = true
2. Call DB::Open() on an empty directory and request to open a few hundred column families
3. Observe that the time needed to open is between 30s - several minutes