Fix: staking pool expirations buckets underflow

[shark0der]

Context

An underflow issue in the StakingPool.getActiveAllocations function was hit during pricing tests.

Changes proposed in this pull request

• The issue was caused by the wrong tranche id passed when calling updateExpiringCoverAmounts from allocate hence having tranches misaligned (stored active cover vs expirations in buckets). This led to getActiveAllocations attempting to expire the tranche that dind't have the allocated capacity.
• This PR also fixes a todo where the indexes seemed wrong inside getActiveAllocationsWithoutCover function.
• Additionally I'm including the commit that exposed a few pricing functions as pure functions. This does not change the logic itself.

Test plan

The test for this is on the test/pricing-unit-tests branch - PR #543 (and it passed) but I'm submitting the fixes separately to avoid any blockers on other PRs.

Checklist

• Rebased the base branch
• Attached corresponding Github issue
• Prefixed the name with the type of change (i.e. feat, chore, test)
• Performed a self-review of my own code
• Followed the style guidelines of this project
• Made corresponding changes to the documentation
• Didn't generate new warnings
• Didn't generate failures on existing tests
• Added tests that prove my fix is effective or that my feature works

Review

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• 🚀 = Feedback

[danoctavian]

LGTM but i can't confidently validate the bug fix right now, some tests are needed at some point