pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ea7b4e9a2a7c23c62278d9a3e0c77310778f62b2

Modified Files
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src/common/parse_manifest.c | 121 +++++++++++++++++++++++++++++++++---
src/include/common/parse_manifest.h | 5 ++
2 files changed, 118 insertions(+), 8 deletions(-)