support audit logging

[dovholuknf]

ziti needs to be able to log important events such as relevant configuration changes. develop some sort of audit facility for things like:

• user logged in
• service/policy/other CRUD type changes
• other 'important' events

[plorenz]

We now have events for CRUD operations, as well as events for many other things (such as sessions). If we need other specific events, we should individual issues for those.