Hosts and Session utility API

Before version 3.11 performing operations on hosts and sessions required to directly reference the hosts, bare_sessions and full_sessions table in the global Metronome’s state, this had the disadvantage to end in unelegant, often duplicated, if statements and to potentially cause tampering of the session states container tables by referencing ’em directly.

To resolve these issues a few new APIs have been introduced and some other modified: