bugfix: add a control_ping() before each update

If VPP were to disconnect either the Stats Segment or the API endpoint,
for example if it crashes and restarts, vpp-snmp-agent will not detect
this. In such a situation, it will hold on to the stale stats and no
longer receive interface updates.

Before each run, send a control_ping() API request, and if that were to
fail (for example with Broken Pipe, or Connection Refused), disconnect
both API and Stats (in the vpp.disconnect() call, also invalidate the interface
and LCP cache), and then fail the update. The Agent runner will then retry
once per second until the connection (and control_ping()) succeeds.

TESTED:
- Start vpp-snmp-agent, it connects and starts up per normal.
- Exit / Kill vpp
- Upon the next update(), the control_ping() call will fail, causing the
  agent to disconnect
- The agent will now loop:
[ERROR   ]      agentx.agent - update         : VPP API: [Errno 1] Sendall error: BrokenPipeError(32, 'Broken pipe'), retrying
[WARNING ]      agentx.agent - run            : Update failed, last successful update was 1673345631.7658572
[INFO    ]     agentx.vppapi - connect        : Connecting to VPP
[ERROR   ]      agentx.agent - update         : VPP API: Not connected, api definitions not available, retrying

- Start VPP again, when its API endpoint is ready:
[INFO    ]     agentx.vppapi - connect        : Connecting to VPP
[INFO    ]     agentx.vppapi - connect        : VPP version is 23.02-rc0~199-gcfaf44020
[INFO    ]     agentx.vppapi - connect        : Enabling VPP API interface events
[DEBUG   ]      agentx.agent - update         : VPP API: control_ping_reply(_0=24, context=12, retval=0, client_index=0, vpe_pid=705326)
[INFO    ]     agentx.vppapi - get_ifaces     : Requesting interfaces from VPP API
[INFO    ]     agentx.vppapi - get_lcp        : Requesting LCPs from VPP API

- The agent resumes where it left off

vppapi.py

@@ -78,6 +78,8 @@ class VPPApi:
         if not self.connected:
             return True
         self.vpp.disconnect()
+        self.iface_dict = None
+        self.lcp_dict = None
         self.connected = False
         return True