multi: fix rpcclient shutdown #16439

Job	Run time
lint code	7m 53s
check commits	5m 23s
cross compilation	31m 58s
check pinned dependencies (google.golang.org/grpc v1.59.0)	7s
RPC and mobile compilation check	4m 19s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	4s
sample configuration check	3m 33s
Sqlc check	2m 41s
run unit tests (btcd unit-cover)	16m 28s
run macOS itest	49m 10s
run unit tests (unit tags="kvdb_etcd")	18m 28s
check release notes updated	2s
run unit tests (unit tags="kvdb_postgres")	17m 22s
run unit tests (unit tags="kvdb_sqlite")	16m 2s
run ubuntu itests (btcd, backend=btcd cover=1)	29m 13s
run windows itest	5m 57s
run unit tests (btcd unit-race)	35m 37s
run ubuntu itests (bitcoind, backend=bitcoind cover=1)	40m 15s
run unit tests (unit-module)	3m 46s
run ubuntu itests (bitcoind-notxindex, backend="bitcoind notxindex")	37m 36s
run ubuntu itests (bitcoind-rpcpolling, backend="bitcoind rpcpolling" cover=1)	37m 51s
run ubuntu itests (bitcoind-etcd, backend=bitcoind dbbackend=etcd)	52m 56s
run ubuntu itests (bitcoind-postgres, backend=bitcoind dbbackend=postgres)	1h 11m 16s
run ubuntu itests (bitcoind-sqlite, backend=bitcoind dbbackend=sqlite)	40m 2s
run ubuntu itests (bitcoind-postgres-nativesql, backend=bitcoind dbbackend=postgres nativesql=true)	1h 11m 56s
run ubuntu itests (bitcoind-sqlite-nativesql, backend=bitcoind dbbackend=sqlite nativesql=true)	40m 59s
run ubuntu itests (neutrino, backend=neutrino cover=1)	35m 52s
finish	4s
	11h 16m 50s

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