A client that opens a session and vanishes before it ever streams (e.g.
OtOpcUa's GalaxyDriver on a failed start) pinned a session slot and a live
x86 worker for the full DefaultLeaseSeconds (1800 s), because detach-grace
only applies to sessions that once had a subscriber.
MxGateway:Sessions:UnattachedSessionGraceSeconds (int, default 0 = off,
validated >= 0) makes the existing lease sweep reap a Ready session that
has never attached an event subscriber once it has been idle (no unary
activity) for the window, with the close reason unattached-grace-expired.
Ready-only, idle-from-last-activity and first-subscriber-cancels keep it
from catching slow starts or unary-only clients.
GatewaySession.ExtendLease had no production callers and did not fit the
rule, so it is removed; tests move leases through TouchClientActivity.
Joseph Doherty
285ef64160