Prevent SharedTree trunk commits from being evicted if they are referenced by branches

[noencke]

Description

This PR updates the EditManager's trunk eviction code to retain all commits on the trunk that are currently the base for any outstanding branch/fork. It attempts to do so while preserving separation-of-concerns by adding additional events to the Branch class, but not putting any trunk-specific code into the Branch class (e.g. sequence numbers). It also:

• Deletes the EditManager module in Core and moves EditManager to be part of SharedTreeCore. This is done since EditManager now has a dependency on Branch, which would otherwise introduce a cyclic dependency. Regardless, it is probably an improvement to the module structure even without the motivation provided by this PR.
• Made some methods on SharedTreeCore protected instead of public to reduce the API surface for eventual/future subclassers of SharedTreeCore (e.g. "other SharedTrees"). These methods are now tested via a test subclass in shared-tree-core/utils.ts

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +32 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	437 KB	437 KB	⯅ +6 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	229.71 KB	229.71 KB	⯅ +2 Bytes
loader.js	160.81 KB	160.82 KB	⯅ +6 Bytes
map.js	44.07 KB	44.07 KB	⯅ +2 Bytes
matrix.js	137.65 KB	137.65 KB	⯅ +2 Bytes
odspDriver.js	92.35 KB	92.36 KB	⯅ +6 Bytes
odspPrefetchSnapshot.js	43.58 KB	43.59 KB	⯅ +4 Bytes
sharedString.js	158.21 KB	158.22 KB	⯅ +2 Bytes
Total Size	1.38 MB	1.38 MB	⯅ +32 Bytes

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Baseline commit: 1f13911

Generated by 🚫 dangerJS against ec656a7

[noencke]

/azp run Build - client packages

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[noencke]

/azp run repo-policy-check

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