Fix sporadically failing connection attempt to server.
Fix sporadically failing connection attempt to server.

With bad timing, a secure channel was "garbage collected" by the UA_Server_cleanup timer (runs every 10 seconds) while the channel was still being established.

jpfr/open62541,JGrothoff/open62541,open62541/open62541,jpfr/open62541,StalderT/open62541,StalderT/open62541,JGrothoff/open62541,StalderT/open62541,JGrothoff/open62541,jpfr/open62541,open62541/open62541,StalderT/open62541,open62541/open62541,jpfr/open62541,JGrothoff/open62541,open62541/open62541