Added VPP state 'NotResponding'

This change allows informing client about the current
VPP state in more detail.

* In case the VPP is overloaded but keeps its configuration,
  client received 'NotResponding' event message
* If the VPP process dies (the socket is closed), the client
  receives 'Disconnected' event message

Additional fix: event messages are discarded if the buffer is
full. This could cause GoVPP to hang after some time in case
the client was not receiveing events.

Change-Id: I94a4647f6643f1d97bf52e5d7996d70229b0577d
Signed-off-by: Vladimir Lavor <vlavor@cisco.com>