Implement SharedTree root local branch as a SharedTreeBranch

[noencke]

Description

The root local branch is currently managed by the EditManager. This PR replaces it with an actual SharedTreeBranch instance, and updates the relevant places to use operations on the branch instance rather than having EditManager and SharedTreeCore re-implement them.

There are many more refactors that "fall out" of this initial refactor. I am not attempting to do all of them here. In fact, just the opposite - I am attempting to refrain from doing much more than the minimal amount of changes that are required here as a first pass. Even so, this PR has quite a few changes. I will follow this up with more refactors in the future to continue to simplify our module boundaries and reduce code.

[msfluid-bot]

⯆ @fluid-example/bundle-size-tests: -2.11 KB

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	446.23 KB	446.23 KB	⯅ +6 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	238.03 KB	238.03 KB	⯅ +2 Bytes
loader.js	155.15 KB	155.16 KB	⯅ +6 Bytes
map.js	46.63 KB	46.63 KB	⯅ +2 Bytes
matrix.js	140.23 KB	140.24 KB	⯅ +2 Bytes
odspDriver.js	92.33 KB	92.34 KB	⯅ +6 Bytes
odspPrefetchSnapshot.js	43.57 KB	43.57 KB	⯅ +4 Bytes
sharedString.js	160.98 KB	160.98 KB	⯅ +2 Bytes
sharedTree2.js	277.8 KB	275.66 KB	⯆ -2.14 KB
Total Size	1.72 MB	1.72 MB	⯆ -2.11 KB

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Baseline commit: f369997

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