garbage collect zero-value UTXOs

[darioAnongba]

Garbage collect the residue orphaned UTXOs when creating transactions. Orphaned UTXOs occur when creating tombstones or full burns.
Currently, these UTXOs accumulate in the DB and are never cleaned. This PR introduces a garbage collection mechanism to collect these UTXOs and use them as inputs of transactions initiated by tapd:

• Transfers
• Burns

The PR adds a new swept flag to the managed_utxo table because UTXOs are not removed from the table when spent. This flag is also returned by the ListUtxos RPC endpoint.

The mechanism preserves the liveness and safety properties, ensuring that zero-value UTXOs can never accumulate in the DB. Adding garbage collection to Mint transactions is not necessary to ensure these properties.

Fixes #514

Note to reviewers

1. UTXO meaning unspent, keeping the spent outputs in a table called manage_utxos seems like a contradiction, same logic applies for the need of a swept flag in that table. We should either rename the table or store spent utxos somewhere else?

[coveralls]

Pull Request Test Coverage Report for Build 18946918516

Details

• 292 of 377 (77.45%) changed or added relevant lines in 16 files are covered.
• 54 unchanged lines in 18 files lost coverage.
• Overall coverage increased (+0.09%) to 56.613%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
asset/asset.go	3	5	60.0%
itest/assertions.go	0	3	0.0%
rpcserver.go	10	15	66.67%
tapfreighter/chain_porter.go	40	45	88.89%
tapfreighter/coin_select.go	23	29	79.31%
tapfreighter/wallet.go	55	76	72.37%
tapdb/assets_store.go	101	144	70.14%

Files with Coverage Reduction	New Missed Lines	%
proof/courier.go	1	79.81%
address/mock.go	2	96.2%
asset/group_key.go	2	72.15%
mssmt/compacted_tree.go	2	77.19%
tapdb/mssmt.go	2	89.55%
tapdb/multiverse.go	2	80.85%
taprpc/universerpc/universe_grpc.pb.go	2	65.45%
universe_rpc_diff.go	2	77.33%
universe/supplyverifier/manager.go	2	64.53%
universe/syncer.go	2	85.5%

Totals
Change from base Build 18942913401:	0.09%
Covered Lines:	64610
Relevant Lines:	114125

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💛 - Coveralls

[ffranr]

Non-blocking, but for the sweepZeroValueAnchorUtxos config field: “sweep orphaned UTXOs” might be clearer. “Anchor” is implied by UTXO, and “zero-value” isn’t accurate since they contain 1000 sats. If they were zero value there would be nothing to sweep.

[darioAnongba]

I like SweepOrphanUtxos better as well. I renamed it everywhere at the config level, let me know if you think we should rename all the way down to the DB layer. In the context of assets, zero-value is correct though, but more verbose and uglier.

[GeorgeTsagk]

last nits, 99% there

[GeorgeTsagk]

LGTM