Optionally provide a callback in apply_changeset

[wonder-sk]

Sometimes it is useful to be able to provide a callback function when applying changes - especially if the input diff file contains many changes. The callback could do things like

• report progress of the operation to the user
• allow early cancellation if the client is impatient
• on server side, yield to indicate that the worker is doing work (did not freeze and should not get killed)

The callback could be something like this:

bool callback(geodiff_context, caller_context, progress);

... and apply_changeset would call it every now and then (not necessarily after each statement it applies).

Should be also supported in python API.

[PeterPetrik]

also possible useful for other function e.g. changeset creation