This adds the ContextGuard struct from the taproot-assets repository
that has been in use there for a while.
The context guard allows for easy creation of contexts that depend on a
central wait group and quit channel.
A context can either be created with a timeout and quit, meaning it will
cancel either on reaching the timeout or when the central quit channel
is closed.
Or a context can be created to block and use a timeout, meaning it will
_not_ cancel on quit but rather block the shutdown until it is completed
(or times out).
The third way is to create a context that just cancels on quit with no
timeout.
