swim: rename public API to 'membership'

[Gerold103]

The logic is the same as with 'raft' - nobody knows what 'swim'/'raft' is. It is simply confusing. Might be renamed to 'membership'. Or something equally general so as not to conflict with the existing 'membership' module which also implements swim.

The old module name could be kept for compatibility for some time. The rename should be trivial - only the string passed to require() must be changed. The API does not mention swim anywhere. Only the module name does.

The sooner we do it, the better and less intrusive it will be. Before people start using swim after its fixed (#5864, #5906).

[kostja]

Such a great idea. And I also admire the false equivalence used to justify it.