so-5.5.17 prepare_receive and prepare_select functions

Usage of ordinary forms of so_5::receive functions inside loops could be inefficient because of wasting resources on construction of internal objects with descriptions of handlers on each receive() call. More efficient way is the preparation of all receive params and reusing them later. A combination of so_5::prepare_receive() and so_5::receive() allows us to do that.

Usage example:

auto prepared = so_5::prepare_receive(
  so_5::from(ch).extract_n(10).empty_timeout(200ms),
  some_handlers... );
...
while( !some_condition )
{
  auto r = so_5::receive( prepared );
  ...
}

Exactly the same situation is for so_5::select functions: some resources are allocated on each call of ordinary so_5::select() function and deallocated on exit. This reallocation can be avoided by using so_5::prepare_select() and appropriate so_5::select() function:

auto prepared = so_5::prepare_select(
  so_5::from_all().extract_n(10).empty_timeout(200ms),
  case_( ch1, some_handlers... ),
  case_( ch2, more_handlers... ),
  case_( ch3, yet_more_handlers... ) );
...
while( !some_condition )
{
  auto r = so_5::select( prepared );
  ...
}