swim: fix a dangerous yield in ffi.gc

FFI can't survive yields. A yield in ffi.C.func() leads to a
crash; yield in ffi.gc is not documented as allowed. Yield in any
GC function leads to garbage collector stuck until the yield is
finished.

This patch makes SWIM GC callback non-yielding. Now yielding
swim_delete() is called in a separate fiber created in GC
callback, but started at the end of event loop only.

Follow up #3234