TreeState support

[LukasKorba]

• @str4d please check this implementation, I believe it's what you were looking for.
• the new service's method getTreeState needs some documentation though

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[str4d]

Reviewed 78cf40f.

[str4d]

getTreeState returns the tree state at the end of a block, and fromState needs to be as of the start of the first scanned block:

Suggested change

	let fromState = try await service.getTreeState(BlockID(height: startHeight))
	let fromState = try await service.getTreeState(BlockID(height: startHeight - 1))

[str4d]

utACK befc5d7