WL#1003: Allow wait_timeout to be unlimited by setting to 0

Affects: Server-7.1   —   Status: Un-Assigned

Currently it is impossible to totally disable wait_timeout from disconnecting an
idle client.

wait_timeout being set to 0 should disable the idle timeout.  Currently setting
it to 0 will instead set it to 1 second, which makes idle connections
disconnected after one second.  No one would use this setting, so it shouldn't
break backwards compatibility to add this feature.

This is very useful when dealing with software which has a connection pool,
since idle clients are expected.  By having them disconnected every so often, it
makes you lose the advantage of connection pooling in the first place.