Add SNMPv3 session support to the asynchronous libraries

[lunkwill42]

ipdevpoll's main entrypoint for SNMP sessions is the AgentProxy class, which is just a thin adapter for pynetsnmp's Twisted-compatible AgentProxy class:

nav/python/nav/ipdevpoll/snmp/pynetsnmp.py

Lines 55 to 57 in 441f4a3

	class AgentProxy(common.AgentProxyMixIn, twistedsnmp.AgentProxy):
	"""pynetsnmp AgentProxy derivative to adjust the silly 1000 value
	limit imposed in getTable calls"""

The main method used to create AgentProxy instances for collection jobs is in nav.ipdevpoll.jobs.JobHandler._create_agentproxy():

nav/python/nav/ipdevpoll/jobs.py

Lines 100 to 133 in ea4f3c5

	def _create_agentproxy(self):
	if self.agent:
	self._destroy_agentproxy()
	if not self.netbox.read_only:
	self.agent = None
	return
	port = next(ports)
	self.agent = AgentProxy(
	self.netbox.ip,
	161,
	community=self.netbox.read_only,
	snmpVersion='v%s' % self.netbox.snmp_version,
	protocol=port.protocol,
	snmp_parameters=snmp_parameter_factory(self.netbox),
	)
	try:
	self.agent.open()
	except SnmpError as error:
	self.agent.close()
	session_count = self.agent.count_open_sessions()
	job_count = self.get_instance_count()
	self._logger.error(
	"%s (%d currently open SNMP sessions, %d job handlers)",
	error,
	session_count,
	job_count,
	)
	raise AbortedJobError("Cannot open SNMP session", cause=error)
	else:
	self._logger.debug(
	"AgentProxy created for %s: %s", self.netbox.sysname, self.agent
	)

Command line programs like navoidverify and naventity seem to have their own versions of this method - perhaps this could benefit from some consolidation now that SNMP session parameters are becoming more complex.