Support for multiple auxiliary channels to a controller

Reviewer: @rlane

Commit Summary:

1) Commit to support creation of multiple aux cxn's based on of_bsn_set_aux_cxns_request
2) Disconnect all the channels incase any channel connected to that controller goes down
3) Initial support to send async messages from switch to controller through main connection, this will be replaced in later commits with a routine to decide based on sender which channel to put the message on.