lib: Add support for libsoup 2.47.3’s new SoupServer API

This is optional, and does not bump the libsoup-2.4 dependency from
2.37.91.
lib: Clear state in an error path

Ensure that the UhmServer’s internal state is unchanged if
uhm_server_start_trace_full() hits an error.
lib: Fix some memory leaks
lib: Rearchitect trace file parsing to use GDataInputStream

Use GDataInputStream rather than a home-grown buffering solution which
broke if the string it was searching for (‘\n> ’ or ‘\n< ’) was split
over a 1024 byte boundary.