The previous implementation of rememberchannelduration (mumble-voip#4147)
acknowledged the unreliability of the last_disconnect database
field. It used a workaround to set the last_disconnect field
to NULL for every user on every start of the mumble server.
However, with a default behavior for NULL, this created the
unintended side-effect that rememberchannel was erroneously
used for every user on their first connection after a server
restart.

This new implementation reverts the workaround of mumble-voip#4147 and
uses the last_active database field to check if last_disconnect
is reliable. If last_disconnect is unreliable, it will now use
the server uptime and compare it to rememberchanneduration.
We expect last_disconnect to be unreliable when the server
was shut down while clients are still connected.

Fixes mumble-voip#5639

(cherry picked from commit ccbf407)