PR27701: debuginfod client: encourage reused debuginfod_client objects

PR27701: debuginfod client: encourage reused debuginfod_client objects

Client objects now carry long-lived curl handles for outgoing
connections.  This makes it more efficient for multiple sequential
queries, because the TCP connections and/or TLS state info are kept
around awhile, avoiding O(100ms) setup latencies.  debuginfod is
adjusted to take advantage of this for federation.  Other clients
should gradually do this too, perhaps including elfutils itself (in
the libdwfl->debuginfod_client hooks).

A large gdb session with 117 debuginfo downloads was observed to run
twice as fast (45s vs. 1m30s wall-clock time), just in nuking this
extra setup latency.  This was tested via a debuginfod intermediary:
it should be even faster once gdb reuses its own debuginfod_client.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>