lib: AgentX: use threads instead of eventloop hack

lib: AgentX: use threads instead of eventloop hack

AgentX fd/timeout handling is rather hackishly monkeyed into thread.c.
Replace with code that uses plain thread_* functions.

NB: Net-SNMP's API rivals Quagga's in terms of age and absence of
documentation.  netsnmp_check_outstanding_agent_requests() in particular
seems to be unused and is therefore untested.

The most useful documentation on this is actually the blog post Vincent
Bernat wrote when he originally integrated this into lldpd and Quagga:
https://vincent.bernat.im/en/blog/2012-snmp-event-loop.html

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>