swim: encapsulate 'uint16' payload size inside swim.c

uint16 was used in public SWIM C API as a type for payload size
to emphasize its small value. But it is not useful in Lua,
because Lua API should explicitly check if a number overflows
uint16 maximal value, and return the same error as in case it is
< uint16_max, but > payload_size_max.

So main motivation of the patch is to avoid unnecessary checks in
Lua and error message duplication. Internally payload size is
still uint16.