test: allow to remove swim nodes from the cluster

Until now it was impossible in swim tests to drop a SWIM instance
from the cluster. It should have been either restarted, or
blocked, but a real drop led to an assertion on any attemp to use
one of methods like swim_wait_timeout(). It was due to inability
to get instance's UUID without the instance itself. Even if it
was stored in membership tables of other instances.

This patch makes swim_cluster store swim instances and UUIDs
separately. This is going to be used to test swim_quit() API.
Also, some cfg parameters are saved as well, like ack timeout, gc
mode. They are used to restart a node with exactly same cfg as
it was before restart. Even if original struct swim * is not
valid already.

Part of #3234