* Use built-in JSON parser for PXF fragments

Instead of relying on a external library, use the built-in JSON
parser in the backend for the PXF fragments parsing. Since this
replaces the current implementation with an event-based callback
parser, the code is more complicated, but dogfooding the parser
that we want extension writers to use is a good thing.

This removes the dependency on json-c from autoconf, and enables
building PXF on Travis for extra coverage.

* Use elog for internal errors, and ereport for user errors

Internal errors where we are interested in source filename should
use elog() which will decorate the error messages automatically
with this information. The connection error is interesting for the
user however, use ereport() instead there.