use Unix Nano timestamp for "now" in poolCommon

[trace-andreason]

The pool’s ticktock goroutine was storing time.Now() in an atomic.Value, which incurs interface boxing and shows up as runtime.convT in profiles. Switch to storing Unix nanos in an int64 with atomic.Load/StoreInt64, and reconstruct time.Time in nowTime(). This preserves behavior while removing the per‑tick allocation and conversion overhead, especially when many pools are created.

1. Are you opening this pull request for bug-fixs, optimizations or new feature?

This is an optimization.

2. Please describe how these code changes achieve your intention.

Each ants pool runs a ticktock() goroutine that periodically caches time.Now() in an atomic.Value. With many pools, the repeated interface boxing and atomic.Value store shows up as runtime.convT in CPU profiles and adds measurable overhead. By switching the cached value to a plain int64 holding Unix nanoseconds and using atomic.StoreInt64/LoadInt64, we keep the same cached‑time behavior for worker expiry but remove the conversion and allocation cost. This reduces per‑pool CPU overhead while preserving existing semantics for idle worker expiration.

3. Please link to the relevant issues (if any).

4. Which documentation changes (if any) need to be made/updated because of this PR?

4. Checklist

• I have squashed all insignificant commits.
• I have commented my code for explaining package types, values, functions, and non-obvious lines.
• I have written unit tests and verified that all tests passes (if needed).
• I have documented feature info on the README (only when this PR is adding a new feature).
• (optional) I am willing to help maintain this change if there are issues with it later.

[panjf2000]

1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.03%. Comparing base (76ddb18) to head (fef4529).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #378   +/-   ##
=======================================
  Coverage   94.03%   94.03%           
=======================================
  Files          14       14           
  Lines         788      788           
=======================================
  Hits          741      741           
  Misses         35       35           
  Partials       12       12           

Flag	Coverage Δ
unittests	94.03% <100.00%> (ø)

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[panjf2000]

lgtm, thx