Make "logging filter imsi " work for (currently) unknown subscribers
osmo-bsc only allows filtered logging base on IMSI it already knows. We should change this so a log filter can also be created for a subscriber that is currently unknown to osmo-bsc.
The attached untested patch should work as intended. It will create the subscriber in the logging filter imsi statement if it doesn't exist. Once we get an CompleteL3 message with the IMSI the code should reuse this subscriber and the logging filter should work.