Implement the charm store logging API.

This will allow storing/retrieving ingestion logs.
To activate the functionality, a client branch is also
required.
Slightly more efficient, because we know these are small
integers, could be:

     mongodocLogLevels = []params.LogLevel{
		mongodoc.InfoLevel:    params.InfoLevel,
		mongodoc.WarningLevel: params.WarningLevel,
		mongodoc.ErrorLevel:   params.ErrorLevel,
     }

You'd then check the array bounds rather than the map
get result. Same thing for mongodocLogTypes.

YMMV.