Adding a way to unregister a channel

Tests are included. The feature will be required when upgrading
tcpsyslog drains from one kind to another -- the drain process itself
will need to transfer the channel property to the drain buffer process,
otherwise messages will be duplicated.
Adding upgrade procedure for tcpsyslog2

There is no direct update to be done between both components, but the
function could be used, for example, if logplex_tcpsyslog_drain decided
to use enter_loop to upgrade.
Replacing the old drains by the new one

Upgrade procedure left to be properly tested.