The description of is_realtime() did not match its actual use. And
client_real_time_priority() did not have any description. It was
hard to come up with good descriptions of what they actually did.
Most backends returned true from is_realtime(), but every one except
JACK used the default implementation of client_real_time_priority() that
returned 0 ... which is exactly the same as when is_realtime() returns
false. Just merge them and keep it simple.
the rest from `tools/convert_boost.sh`.
* replace boost::function, boost::bind with std::function and std::bind.
This required some manual fixes, notably std::placeholders,
some static_casts<>, and boost::function::clear -> = {}.
This is mostly a simple lexical search+replace but the absence of operator< for
std::weak_ptr<T> leads to some complications, particularly with Evoral::Sequence
and ExportPortChannel.
