Invalidate msgTable map during reconnect

[themiron]

It seems possible that control_ping msg id gets reused too early after reconnection to vpp, and once vpp api gets changed stored msg id is no longer valid and can crash vpp. Fix that with dropping msgTable on disconnect and protect map access with mutex.

Example:

  $ gdb ...
  #19 0x00007f63fd53d6d4 in msg_handler_internal (am=0x7f63fd76eb20 <api_global_main>, the_msg=0x7f62cb8f68b0, trace_it=0, do_it=1, free_it=0)
  (gdb) p id
  $1 = 590
  (gdb) p am->msg_names[id]
  $2 = 0x703900 "get_node_index"

  $ sudo vppctl sh api message-table | grep 590
  590  control_ping

[themiron]

@ondrej-fabry hi, kindly please take a look

[ondrej-fabry]

@themiron LGTM. Thanks for the PR.

[themiron]

thank you