Improve trunk eviction in EditManager

[noencke]

Description

The SharedTree EditManager's trunk commit eviction was both more complicated than necessary and also buggy. This PR cleans up the code and addresses the known bugs. Previously, when the minimum sequence number advanced to some sequence number M, all commits with sequence numbers below M were dropped, but a commit with exactly sequence number M was not because any number of branches might be based off of that commit (e.g. the local branch), and dropping it was impossible because a branch might continue to reference it. After this PR, that commit is still not dropped from memory, however it takes the place of the special "trunk base" commit or "origin" commit which precedes the trunk but is not considered part of the trunk. Not only does this remove multiple edge cases from the eviction logic (and the bugs that currently dwell therein), it also makes it more intuitive to predict which commits are removed from the trunk as the minimum sequence number advances.

This PR also does some minor cleanup and optimizations in EditManager, adds some more eviction test cases, and enables an eviction pass after tracked branches rebase (which will evict the trunk more aggressively in scenarios where a long-lived user-created branch repeatedly rebases over a parent branch).

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +12 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	436.74 KB	436.74 KB	■ No change
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	236.57 KB	236.57 KB	■ No change
loader.js	146.52 KB	146.52 KB	■ No change
map.js	45.98 KB	45.98 KB	■ No change
matrix.js	138.31 KB	138.31 KB	■ No change
odspDriver.js	88.94 KB	88.94 KB	■ No change
odspPrefetchSnapshot.js	41.75 KB	41.75 KB	■ No change
sharedString.js	154.72 KB	154.72 KB	■ No change
sharedTree2.js	231.48 KB	231.49 KB	⯅ +12 Bytes
Total Size	1.64 MB	1.64 MB	⯅ +12 Bytes

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Baseline commit: 19c99b7

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