dashboard: share Spanner clients #19161

Job	Run time
old	7m 6s
dashboard	2m 7s
build	3m 8s
aux	3m 59s
gvisor	4m 4s
arch (presubmit_arch_netbsd)	3m 55s
race_dashboard	2m 43s
arch (presubmit_arch_darwin)	3m 55s
arch (presubmit_arch_linux)	10m 48s
arch (presubmit_arch_freebsd)	5m 58s
arch (presubmit_arch_openbsd)	3m 44s
arch (presubmit_arch_windows)	3m 19s
arch (presubmit_arch_executor)	3m 22s
race	7m 25s
	1h 5m 33s

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