feat(renterd): bulk rescan hosts #514

Job	Run time
release-docker	15m 47s
release	12m 44s
test-go (ubuntu-latest, ./renterd)	46s
test-js	3m 58s
test-go (ubuntu-latest, ./hostd)	52s
test-go (ubuntu-latest, ./walletd)	46s
test-go (macos-latest, ./renterd)	53s
test-go (macos-latest, ./hostd)	47s
test-go (macos-latest, ./walletd)	50s
test-go (windows-latest, ./renterd)	1m 31s
test-go (windows-latest, ./hostd)	1m 30s
test-go (windows-latest, ./walletd)	1m 30s
e2e / explorer-e2e-shard-tests	0s
e2e / hostd-e2e-shard-tests	0s
e2e / renterd-e2e-shard-tests	0s
e2e / walletd-e2e-shard-tests	0s
e2e / explorer-e2e-merge-reports / merge-reports	5s
e2e / hostd-e2e-merge-reports / merge-reports	6s
e2e / renterd-e2e-merge-reports / merge-reports	9s
e2e / walletd-e2e-merge-reports / merge-reports	9s
	42m 23s

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