Implement concurrent session verifier

The following needs to be implemented in the scope of this JIRA:

• we need 4 new Gateway-level configurations:
  • privileged user list (defaults to an empty collection)
  • non-privileged user list (defaults to an empty collection)
  • session limit for privileged users (defaults to 3)
  • session limit for non-privileged users (defaults to 2)
• if a session limit for any of the groups is set to a negative number, that means the users in that group are allowed to have an unlimited number of sessions
• In addition to the new configs, a verifier has to be implemented that enforces the following business logic: if a user is listed in the above-introduced privileged/non-privileged collection AND is about to pass a configured session limit the verification should fail. The verification should succeed if the given user is declared neither a privileged nor a non-privileged user.

The new verifier implementation may be placed in the gateway-spi-common project for now.