Add support for incrementally parsing backup manifests

Add support for incrementally parsing backup manifests

This adds the infrastructure for using the new non-recursive JSON parser
in processing manifests. It's important that callers make sure that the
last piece of json handed to the incremental manifest parser contains
the entire last few lines of the manifest, including the checksum.

Author: Andrew Dunstan
Reviewed-By: Jacob Champion
Discussion: https://postgr.es/m/7b0a51d6-0d9d-7366-3a1a-f74397a02f55@dunslane.net