Proper interface for informative messages and event logging

[ieslick]

Currently we print, signal or do other things when various system events occur. We should turn this into an appropriate log-level notification system where the user can specify the informativeness they want. Warnings and above should be printed by default to help the user avoid shooting themselves in the foot.

Do we want to also add basic event logging such as: High level: (created pobj of <class>: oid = foo) (created new repository <spec>) Low level: (wrote <key>,<value> to <btree:oid>) (deleted <key> from <btree:oid>) (slot-access to <obj:oid:class> => <value>)

This could be done easily as around or after methods on the major interfaces, perhaps enabled as a reader option (rather ugly, but performance enhancing). We could also just insert code into the appropriate places.