Add TreeStatus.Created and tighten transaction APIs

[noencke]

BREAKING CHANGE: A new tree status has been added for SharedTree nodes.

TreeStatus.Created indicates that a SharedTree node has been constructed but not yet inserted into the tree.
Constraints passed to the runTransaction API are now marked as readonly.

[yann-achard-MS]

This PR should be described as breaking change (and never be cherry-picked into earlier RCs)

[yann-achard-MS]

Is there are reason why this PR includes the readonly-fication changes at the same time as the introduction of the "Created" status?

[noencke]

Is there are reason why this PR includes the readonly-fication changes at the same time as the introduction of the "Created" status?

There's no particular reason.

[yann-achard-MS]

Is there are reason why this PR includes the readonly-fication changes at the same time as the introduction of the "Created" status?

There's no particular reason.

Okay, that's a little weird but the changes are simple enough that it doesn't bother me.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +30 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	453.27 KB	453.27 KB	■ No change
azureClient.js	552.5 KB	552.5 KB	■ No change
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	256.97 KB	256.97 KB	■ No change
fluidFramework.js	360.46 KB	360.48 KB	⯅ +15 Bytes
loader.js	134.38 KB	134.38 KB	■ No change
map.js	41.46 KB	41.46 KB	■ No change
matrix.js	143.75 KB	143.75 KB	■ No change
odspClient.js	520.87 KB	520.87 KB	■ No change
odspDriver.js	97.3 KB	97.3 KB	■ No change
odspPrefetchSnapshot.js	42.16 KB	42.16 KB	■ No change
sharedString.js	160.27 KB	160.27 KB	■ No change
sharedTree.js	360.45 KB	360.46 KB	⯅ +15 Bytes
Total Size	3.2 MB	3.2 MB	⯅ +30 Bytes

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Baseline commit: 9bf98f0

Generated by 🚫 dangerJS against 451c485