gate: protect against calling close() more than once
gate: protect against calling close() more than once

Current implementation doesn't allow calling close() multiple times.
Complain loudly if someone tries to do that.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>
Message-Id: <1454504082-18081-1-git-send-email-e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

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