Simplify/Refactor BranchCommitEnricher

Description

This tightens the state machine space for BranchCommitEnricher and makes various simplifications to the API and code.

The API surface of BranchCommitEnricher is reduced to functions that:
1. Process branch changes
2. Get enriched commits
3. Acknowledge transaction state transitions
Item (i) used to be made of a few different methods whose invocations needed to be coordinated by SharedTreeCore. Now, BranchCommitEnricher manages this internally. Previously, it needed to know whether or not the commit being submitted was the commit of the outermost transaction. This could be problematic for future refactors of SharedTreeCore, so the enricher now derives that information from its internals (since it already keeps track of ongoing transactions via the TransactionEnricher.
The map of processed commits stored in the enricher is now a WeakMap, meaning that the enricher does not need to worry about accidentally holding on to commits forever. The logic that "purges" map entries between changes has been removed since the entries will now be dropped automatically as the commits become unreferenced.
TransactionEnricher no longer allows getting the enriched change in the middle of a transaction.
We now properly throw an error if a the SharedTree is attached in the middle of a transaction - we do not and cannot currently support this scenario.
Various renames, conditional refactors, inlining, etc. to shorten lines, improve code flow, simplify conditional logic, etc. and additional documentation.

⯅ @fluid-example/bundle-size-tests: +2.41 KB

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	467.24 KB	467.27 KB	⯅ +35 Bytes
azureClient.js	564.01 KB	564.06 KB	⯅ +49 Bytes
connectionState.js	724 Bytes	724 Bytes	■ No change
containerRuntime.js	263.43 KB	263.45 KB	⯅ +14 Bytes
fluidFramework.js	427.22 KB	428.32 KB	⯅ +1.1 KB
loader.js	134.18 KB	134.19 KB	⯅ +14 Bytes
map.js	42.71 KB	42.71 KB	⯅ +7 Bytes
matrix.js	150.15 KB	150.16 KB	⯅ +7 Bytes
odspClient.js	529.85 KB	529.89 KB	⯅ +49 Bytes
odspDriver.js	97.88 KB	97.9 KB	⯅ +21 Bytes
odspPrefetchSnapshot.js	42.81 KB	42.83 KB	⯅ +14 Bytes
sharedString.js	166.23 KB	166.24 KB	⯅ +7 Bytes
sharedTree.js	417.68 KB	418.77 KB	⯅ +1.09 KB
Total Size	3.38 MB	3.38 MB	⯅ +2.41 KB

Baseline commit: 421ba403750ecf86da0490a9805bf2bef8573243

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