Use incremental parsing of backup manifests.
This changes the three callers to json_parse_manifest() to use
json_parse_manifest_incremental_chunk() if appropriate. In the case of
the backend caller, since we don't know the size of the manifest in
advance we always call the incremental parser.
Author: Andrew Dunstan
Reviewed-By: Jacob Champion
Discussion: https://postgr.es/m/
7b0a51d6-0d9d-7366-3a1a-
f74397a02f55@dunslane.net