Port akka-core#31834: Add ScheduledClock to reduce nanoTime overhead in recency-based passivation

[pjfanning]

part of #2730

Recency-based passivation strategies (the new default) call System.nanoTime() on every message. This introduces a ScheduledClock that refreshes the timestamp on a configurable interval (default 1s) and increments a counter between updates — eliminating per-message nanoTime calls under high throughput.

New: Clock actor extension (org.apache.pekko.util)

• Clock — Extension trait with currentTime(): Long and earlierTime(duration): Long
• NanoClock — direct System.nanoTime() delegate (used when scheduled-clock-interval = 0)
• ScheduledClock — updates via scheduler at the configured interval; increments atomically between updates so time never goes backwards

Config

pekko.scheduled-clock-interval = 1 s  # set to 0 to use System.nanoTime() directly

Internal API changes

• RecencyList — removed inner Clock/NanoClock; added apply(clock: Clock) factory
• FrequencyList.withOverallRecency.empty and SegmentedRecencyList.withOverallRecency.empty — now take an explicit Clock parameter instead of constructing a NanoClock internally
• EntityPassivationStrategy.apply — signature changed to accept clock: () => Clock; all strategy and replacement-policy constructors propagate the clock
• Shard — passes () => Clock(context.system) to EntityPassivationStrategy
• Simulator — updated to pass clock factory through to all strategy creators

Tests

• TestClock — shared controlled clock extracted from per-spec companion objects
• ScheduledClockSpec — covers scheduled update, per-call increment, and non-regression (time never decreases under concurrent access)
• Passivation specs — replaced System.nanoTime() with clock.currentTime() for consistency; test config sets pekko.scheduled-clock-interval = 100 ms

[He-Pin]

nice to have