Defer txt record lookup until connection attempt

Don't try to resolve _xmppconnect records for servers before trying to connect.
Prevent warnings from gajim.c.resolver for accounts with "connect on startup" disabled.
3 jobs for feature/deferTXTresolverUntilConnect in 1 minute and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #2937
run-pylint

00:01:34

passed #2936
run-test

00:00:03

 
  Build
passed #2938
run-build

00:00:07