Replace replace_executor with use of require

[jaredhoberock]

Now that we have require, the additional customization point replace_executor may be unnecessary.

.on() could instead work like this:

template<class OtherExecutor>
auto on(const OtherExecutor& ex) const
{
  // require the current executor's bulk guarantee
  auto replacement = require(ex, bulk, query(this->executor(), bulk_guarantee))
  return {replacement};
}

We should be able to eliminate replace_executor and its customizations this way.