fix: use sync pool for strategy randomness

[sighphyre]

Instead of blocking on a single shared Mutex, randomness now uses a sync pool, which gives multiple goroutines a bit more breathing room to get access to randomness without lining up one behind another

[coveralls]

Pull Request Test Coverage Report for Build 19563565440

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 84 of 127 (66.14%) changed or added relevant lines in 6 files are covered.
• 67 unchanged lines in 7 files lost coverage.
• Overall coverage decreased (-4.5%) to 77.151%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
api/feature.go	4	6	66.67%
internal/strategies/flexible_rollout.go	0	2	0.0%
internal/strategies/helpers.go	0	10	0.0%
metrics.go	72	101	71.29%

Files with Coverage Reduction	New Missed Lines	%
nooplistener.go	2	62.5%
internal/api/metrics.go	3	0.0%
debuglistener.go	6	0.0%
unleash_mock.go	6	75.0%
client.go	7	86.93%
feature_state.go	20	59.29%
metrics.go	23	75.37%

Totals
Change from base Build 19538258348:	-4.5%
Covered Lines:	1587
Relevant Lines:	2057

---

💛 - Coveralls

[FredrikOseberg]

This looks sane. Nice! Also this is backwards compatible so we can merge it towards both branches.

EDIT: Is the target branch here correct Simon? I would imagine you want to merge this into v5 or v6.

[sighphyre]

This looks sane. Nice! Also this is backwards compatible so we can merge it towards both branches.

EDIT: Is the target branch here correct Simon? I would imagine you want to merge this into v5 or v6.

Yessir, it's correct. This is a stacked PR, if you're not familiar with the flow, it's worth doing some reading on it because it's a hella useful flow for this kind of situation. This entire chain builds on top of itself so this entire set of PRs require the work before them. Stacking the PR lets me work without being blocked by review and prevents you from having to review a 4k +/- monstrosity

The base now points to v6, so as these merge they'll all fall into that