Replace logOp() with a more operation-aware observer interface

Rather than having a single logOp() function for all types of data and metadata operations that might be observed, we will build an observer interface for such operations.

Instead of logOp("i", ...) there will be OpObserver::onInsert(...), e.g.

There will be one instance of the observer in the database server, with the observation actions statically encoded. This ticket is not for allowing the dynamic registration of data and meta-data event observers.