Remove session expiration timer upon setting up bridge connection (fix #2634)

[swiergot]

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This fixes #2634 by removing a possibly existing session expiration timer when setting up a bridge connection.

The bug is a side effect of the fix to #2546. Normally session expiration timers are not created for bridge connections. Starting with 2.0.15 the timers are created at startup for all clients found in the persistent database which also has records for bridge connections.

I have not worked with mosquitto code before, but I can see a few potential solutions:

• Don't create session expiration timers for bridge connections found in the persistent database. Unfortunately, there is no way to recognize bridge connections at that point. It would require an additional flag in the database.
• When a bridge connection is being configured and a context for the client ID already existed, remove the corresponding session expiration timer.
• Don't write bridge connections to the persistent database. I'm not completely sure but I think those records are useless for bridge connections anyway (although apparently their existence in the persistent database was a conscious decision).
• Ignore bridge connections when checking session expiration timers.

I have decided to go with option number 2.

[swiergot]

Nevermind, apparently the fix (6d240a9) has been on the fixes branch for over 3 months. There just hasn't been any new release since.