Debug and fix `listunspent` #631

Job	Run time
windows itest	28m 45s
macOS itest	17m 27s
basic itests (btcd, backend=btcd cover=1)	13m 7s
itests (bitcoind-rpcpolling, backend="bitcoind rpcpolling")	22m 44s
basic itests (bitcoind, backend=bitcoind cover=1)	15m 41s
itests (bitcoind-etcd, backend=bitcoind dbbackend=etcd)	29m 39s
basic itests (bitcoind-notxindex, backend="bitcoind notxindex")	14m 5s
itests (bitcoind-sqlite, backend=bitcoind dbbackend=sqlite)	23m 51s
basic itests (neutrino, backend=neutrino cover=1)	13m 59s
itests (bitcoind-sqlite-nativesql, backend=bitcoind dbbackend=sqlite nativesql=true)	23m 54s
itests (bitcoind-postgres, backend=bitcoind dbbackend=postgres)	41m 21s
itests (bitcoind-postgres-nativesql, backend=bitcoind dbbackend=postgres nativesql=true)	42m 5s
	4h 46m 38s

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