controller: finish remote controller when all session finished #2686

Job	Run time
buildkit-edge	2m 32s
prepare-binaries	7s
prepare-test	2m 11s
binaries (darwin/amd64)	2m 13s
binaries (darwin/arm64)	2m 5s
binaries (linux/amd64)	2m 8s
binaries (linux/arm/v6)	2m 3s
binaries (linux/arm/v7)	2m 45s
binaries (linux/arm64)	2m 32s
binaries (linux/ppc64le)	2m 3s
binaries (linux/riscv64)	2m 24s
binaries (linux/s390x)	2m 13s
binaries (windows/amd64)	2m 7s
binaries (windows/arm64)	2m 30s
test (docker, ./tests)	4m 35s
test (docker\+containerd, ./tests)	3m 58s
test (docker-container, ./tests)	3m 48s
test (remote, ./tests)	3m 39s
test (./..., 1)	4m 37s
release	24s
bin-image	0s
	50m 54s

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