dns_journal_iter_init() can now return the size of the delta

dns_journal_iter_init() can now return the size of the delta

the call initailizing a journal iterator can now optionally return
to the caller the size in bytes of an IXFR message (not including
DNS header overhead, signatures etc) containing the differences from
the beginning to the ending serial number.

this is calculated by scanning the journal transaction headers to
calculate the transfer size. since journal file records contain a length
field that is not included in IXFR messages, we subtract out the length
of those fields from the overall transaction length.

this necessitated adding an "RR count" field to the journal transaction
header, so we know how many length fields to subract. NOTE: this will
make existing journal files stop working!