Remove the global variable for connection

Remove the global variable for connection

- this also fixes issue where after connect failure all following connects
  failed 'with only one connection per process is supported'
- this global was used to limit to single connection, but the actual
  limitation is for the adapter and with different adapter implementation
  multiple connections could actually be supported
- the global defined in the vppapiclient package will continue to limit
  to single connection via VppClient implementation of the adapter

Change-Id: Id06dd6e80b51d1d43f6f75bbcc6e01c6f21c605a
Signed-off-by: Ondrej Fabry <ofabry@cisco.com>