[apurtell]

When the C client received a reply to a SetAuth request, auth_completion_func ran every pending callback with the status of that one reply without checking which credential the reply belongs to. The client also put SetAuth frames at the front of the send queue, so frames from back-to-back calls went out in reverse order.

There were additional related problems on the same path:

• Credentials that the client sends again during a reconnect could complete newly issued zoo_add_auth calls before the server checked them.
• An auth reply on a handle with no credentials dereferenced a NULL list and crashed the I/O thread.
• Concurrent zoo_add_auth calls could both send the last credential in the list.
• If a frame could not be queued zoo_add_auth would return an error but leave the callback registered.

Each handle now keeps a FIFO of pending SetAuth frames in the order of frames on the wire. Each reply completes only the entry at the head.

Related fixes:

• An auth reply with no pending frame is logged and ignored. Before, it dereferenced a NULL credential list.
• zoo_add_auth holds the auth lock while it queues the frame of its own credential. Concurrent calls no longer send the tail credential twice. The frame goes to the end of the send queue.
• zoo_add_auth checks its allocations. If the frame cannot be queued, the credential is not added and its completion never runs.
• zoo_add_auth no longer sends a frame in ZOO_ASSOCIATING_STATE. Session establishment sends the credential with its completion.

Adds the Zookeeper_authCompletion suite in tests/TestAuthCompletion.cc.