fix: computeIfAbsent should not update write timestamp for existing entries

[daguimu]

Problem

LocalCache.computeIfAbsent() incorrectly resets the write timestamp when called on an existing key, even though the value is not changed. This causes expireAfterWrite caches to behave like expireAfterAccess caches — entries never expire as long as they are accessed via computeIfAbsent() at intervals shorter than the write expiry duration.

This is particularly problematic for Spring Boot applications using @Cacheable with synchronized access, which delegates to computeIfAbsent() internally.

Root Cause

computeIfAbsent() delegates to Segment.compute(). When the computed value is the same reference as the existing value (newValue == valueReference.get()), the code path at LocalCache.java:2278 called recordWrite(e, 0, now), which resets the write timestamp via entry.setWriteTime(now). This incorrectly extends the expireAfterWrite deadline.

Fix

Replaced the recordWrite(e, 0, now) call with inline logic that:

• Updates the access time (for expireAfterAccess correctness)
• Re-adds the entry to both accessQueue and writeQueue (required because they were removed earlier in the compute() method at lines 2251-2252)
• Does NOT reset the write timestamp, preserving the original expireAfterWrite deadline

Tests Added

Change Point	Test
Do not reset write time when value unchanged	testComputeIfAbsent_existingEntryWriteTimeNotChanged() — verifies getWriteTime() is unchanged after computeIfAbsent on existing key
Entry properly expires despite repeated computeIfAbsent	testComputeIfAbsent_existingEntryExpiresAfterWrite() — verifies entry expires after write duration even with repeated computeIfAbsent calls
Regression: compute() with new value still updates write time	testCompute_returningNewValueUpdatesWriteTime() — verifies that compute() returning a new value still correctly updates write time

Impact

Only affects the code path in Segment.compute() where the computed value is the same reference as the existing value. This path is exercised by computeIfAbsent() on existing keys, and also by compute()/computeIfPresent()/merge() when their function returns the existing value unchanged. The new-value and removal paths are unaffected.

Fixes #3700