[MTRDevice] Defer first Thread subscription until operational advertisement seen

Avoid racing the very first SubscribeRequest against threadradiod's
[WED_START] on Thread cold-start, which would otherwise produce a
No-route-to-host failure and burn 3-6s of coldstart on a retry.  When
this is the first subscription attempt for a Thread device and we have
no cached IP address, defer the subscribe until either an operational
mDNS advertisement arrives or a 1-second watchdog fires.

Short-circuits the gate when we already have evidence the device is
reachable (cached IP, recent in-memory report, recent persisted-report
hint from a prior process), and bypasses entirely on the explicit
sDeviceMayBeReachableReason signal.  The watchdog is stored as a
cancellable dispatch_block_t so invalidate/controllerSuspended/the
advertisement-driven cancel path can drop its captures immediately
instead of holding them for up to a full second.

NSUserDefaults writes for the cross-process recency hint are funnelled
through a dedicated serial queue so an invalidate-time clear is
guaranteed to observe and overwrite any in-flight write (otherwise an
async setObject: could land after the clear, leaving stale state that
poisoned the next test run).  Tests wipe stale
MTRDeviceMostRecentReportTime-* keys in tearDown for the same reason.

Author: woody-apple