fix(sentinel): back off between failed topology refreshes

[FZambia]

Summary

refreshRetry retried failed topology refreshes with no delay and never exited on Close. This adds equal-jitter backoff and a prompt shutdown.

Impact

• When every sentinel is unreachable (a failover that takes the primary and a co-located sentinel down together), refreshRetry spins as fast as the CPU allows, dialing every sentinel on each iteration — one client can saturate a core and flood the surviving sentinels with connection attempts exactly while they are running the election.
• A client closed during this leaks the goroutine: the loop never checked stop, so it retried forever.

Root cause

refreshRetry was an unbounded goto retry loop with no delay and no shutdown check.

Fix

Equal-jitter exponential backoff between attempts (1ms base, 1s cap), and return once Close() has been called. Still retries indefinitely — only the spin is removed.

Tests

• TestRefreshRetryDelay — the delay is always positive, never exceeds the cap, and grows with attempts.
• TestSentinelRefreshRetryBackoff — pins both properties under the leak detector: no spin (≤ a few dozen attempts in 200ms, vs hundreds of thousands before) and prompt exit on Close.

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Note

Medium Risk
Touches sentinel failover/reconnect behavior under total sentinel outage; mis-tuned backoff could slow recovery, but changes are localized and heavily tested.

Overview
Replaces the sentinel client’s refreshRetry tight retry loop with equal-jitter exponential backoff between failed topology refreshes, so unreachable sentinels during failover are not hammered with CPU-bound dial storms.

refreshRetry now loops while c.stop is clear: on failure it sleeps using refreshRetryDelay (ms-scale base, 1s cap, jitter preserved at the cap) and returns on success or Close(). waitBeforeRetry breaks out of long sleeps in 20ms steps when stop is set.

New tests lock in delay bounds/growth/jitter, interruptible wait on close, and that failed refresh no longer spins while the goroutine exits after Close().

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