swim: introduce member reference API

Struct swim_member pointer is used to learn member's status,
payload, incarnation etc. To obtain a pointer, a one should
either lookup the member by UUID, or obtain from iterators API.
The former is long, the latter is useless when a point lookup is
needed.

On the other hand, it was not safe to keep struct swim_member
pointer for a long time, because it could be deleted at any
moment.

This patch allows to reference a member and be sure that it will
not be deleted until dereferenced explicitly. The member still
can be dropped from the member table, but its memory will be
valid. To detect that a member is dropped, a user can use
swim_member_is_dropped() function.

Part of #3234