database: wait for asynchronous operations to end before closing CF
database: wait for asynchronous operations to end before closing CF

This was part of "add gate for generic async operations to column family" but
somehow didn't make it into the final patch.

Add the missing piece.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <20170830164205.4497-1-7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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