The per-command V8 watchdog kept a single global slot whose correctness relied
on the old process-wide lock (at most one command armed at a time). With
thread-per-connection several connections arm concurrently, so a second arm
overwrote the first's slot and left that command's isolate unbounded. Key the
armed slots by generation so each connection's command has its own; the worker
fires whichever have overrun and terminates each via its IsolateHandle.
Refs #430.
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