Add utility to get statistics of the underlying database

[muhrin]

It would be useful to have a utility that can output statistics about the current database, including:

• The number of each type of object
• The size of all obejcts of particular type
• The size of the history

It may be useful to have this as a cli tool as well just for basic diagnostics. Furthermore, it would be useful if it were extendable to be able to include other metrics that crop up.

[muhrin]

This could come as a cli command e.g. mince stats