allow dns_journal_rollforward() to read old journal files

when the 'max-ixfr-ratio' option was added, journal transaction headers were revised to include a count of RR's in each transaction. this made it impossible to read old journal files after an upgrade.

this branch restores the ability to read old journal files, so that upgrades can run smoothly. if a journal cannot be rolled forward using the new transaction header format, we try again with the original format.

Closes #2505 (closed)