Chore/Rename bucketStartTime to bucketEndTime

[0xdewy]

Context

Closes #388 To align the naming between start/end, when expiring buckets and tranches.

Changes proposed in this pull request

This PR renames bucketStartTime to bucketEndTime to match the tranche expiry.

Test plan

Just a semantic change, no tests were added.

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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[shark0der]

The first bucket and the first tranche (ie bucket id and tranche id = 0) start at zero unix time.

When you deposit to the tranche zero, your deposit is locked while the tranche is active. The tranche zero starts at trancheStartTime = trancheId * TRANCHE_DURATION = 0 and expires at trancheEndTime = (trancheId + 1) * TRANCHE_DURATION = TRANCHE_DURATION. That's why we're using trancheEndTime in our code, not start time.

For the buckets, let's say that current unix time = 0. Say someone buys a cover for a period equal to exactly one bucket. We increase the current reward per second and write down that we should decrease the reward per second at a particular time in the future. That particular time is when the bucket id = 1 starts. For this reason we're using bucketStartTime = bucketId * BUCKET_DURATION.

So technically just renaming bucketStartTime to bucketEndTime is incorrect, because that's not what it represents. If we want to use bucketEndTime, we will need to reflect this change in the code as well, and use bucketEndTime = bucketId * BUCKET_DURATION where bucketId will be the expiring bucket id BUT ALSO on each cover buy store the reward amount to be reduced in the last bucket that the cover will be active, instead of the first bucket where the cover is expired.