Those flags are meant for detecting which socket has changed, so in the
future we can have more granular updates.
Node now stores an update_flags member which is modified every time
a socket is changed though Node::set. The flags are or-able bits
stored in SocketType instances. Each SocketType stores a unique bit
out of 64, for the 64 bits of an uint64_t; the bit
corresponds to the index of the socket in the Node's sockets array +
1, so the socket at index 5 will have the 6th bit set as its flag. This
limits us to 64 sockets per Node, which should be plenty for the current
set of Nodes that we have.
This does not change the behavior of other parts of Cycles.
This is part of T79131.