Abstract updates into a vtable

Abstract updates into a vtable

This commit adds a layer of indirection to the apply_diff logic used by
IXFR and resigning by having the database updates go through a vtable.

We do this in three steps:
 - We extend dns_rdatacallbacks_t vtable to allow subtraction and
   resigning.
 - We add a new set of api (begin|commit|abort)update to the dbmethods
   vtable, that model an incremental update that can be aborted.
 - We extract the core logic of diff_apply into a function that
   satisfies the new interface.
 - We make diff_apply use this new function, and log the results.

The intent of this commit is to allow databases to expose a batch
incremental update implementation, just like they expose a custom
batch creation implementation through (begin|end)load.