swim: cache members in Lua member table

Each time a member was returned from a SWIM instance object, it
was wrapped by a table with a special metatable, cached payload.

But next the same lookup returned a new table. It

  - created garbage as a new member wrapper;
  - lost cached decoded payload.

This commit caches in a private table all wrapped members and
returns an existing wrapper on a next lookup. A microbenchmark
showed, that cached result retrieval is 10 times faster, than
each time create a new table.

Cache table keeps week references - it means, that when a member
object looses all its references in a user's application, it is
automatically dropped from the table.

Part of #3234