Fix potential incorrect destruction of channels.

utcp_abort_all_connections() was called when a node is blacklisted or a
a node reconnects with a different session ID. In both cases, all active
channels should be closed. However, utcp_abort_all_connections() also freed
the UTCP connection object, which would cause a double free when the
application freed the corresponding meshlink_channel object.

Rename utcp_abort_all_connections() to utcp_reset_all_connections(), and
have it not invalidate any handles.