Subscribe to DIDs that come into wallet

[Quexington]

This addresses #17242.

There's sort of two fixes in this PR. The first subscribes to any DIDs that we sync so that when they are spent we still get their update. This is normally covered by the transactions we submit but in edge cases where transactions are not actually pushed to the wallet, we can forget to mark previous DIDs as spent.

The second fix is to make the DIDWallet.get_coin() method more robust to make sure that if we somehow get into a state where multiple DIDs are unspent, we take the unspent one with the highest confirmed height to drastically increase our chances of selecting the proper one. Should be an unnecessary change but if future bugs arise, this will mute the symptoms a bit.

[coveralls-official]

Pull Request Test Coverage Report for Build 7491404410

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• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 90.587%

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Totals
Change from base Build 7490775207:	0.009%
Covered Lines:	94573
Relevant Lines:	104370

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