Design and implement a GSS-TSIG key manager
Next step after #1960 (closed):
- extend the GSS-TSIG key class with parameters from the GSS-TKEY exchange
- add a list of GSS-TSIG keys in the DNS server class
- create a manager which periodically setups GSS-TSIG keys
- when #2011 (closed) will be merged (note it is a core code ticket when this one is for premium code) write a handler for the new callout
- investigate the shutdown issue (IOFetchs can be cancelled and it is not critical i.e. we can drop GSS-TSIG keys (vs prending DNS updates))
Most activities should be done as callbacks on the I/O service including the start/launch method.