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.
This allows to pass any GraphChain to the Graph to process.
It removes the need to use a mutex to swap two dedicated
chains (setup-chain <> active-chain, pending-chain).
Also various special cases pertaining to graph interaction
while auditioning and route-deletion can be removed.
This also unconditionally creates a graph-thread for GraphChains
to be processed, even if the main callback uses a special-cased
sorted RouteList if there is only one process thread.
The process-graph should only be concerned with GraphNodes,
which may or may not be Routes.
This also removes intrinsic connection information from
the graph-node. Connection information is to be kept separate
from the nodes.
When the graph is re-calculated in the background, old information
has to be retained until the new graph becomes active.
Previously *new* information was already stored in the nodes
while the graph is sorted, even though the new graph was not
active.
"While 'atomic' has a volatile qualifier, this is a historical
artifact and the pointer passed to it should not be volatile."
Furthermore "It is very important that all accesses to a
particular integer or pointer be performed using only this API"
(from https://developer.gnome.org/glib/2.68/glib-Atomic-Operations.html)
Hence initialization of atomic variables is changed to also use
this API, instead of directly initializing the value.
This also fixes a few cases where atomic variables were
accessed directly.
see also libs/pbd/pbd/g_atomic_compat.h
