add grpc concurrency limit #476

Job	Run time
Check misspelled words	13s
Go build for different platforms	8m 49s
Go build with -tags=stringlabels	2m 15s
Linters (Static Analysis) for Go	3m 43s
Thanos end-to-end tests (8, 0)	5m 38s
Thanos unit tests	12m 43s
Thanos end-to-end tests (8, 1)	4m 35s
Thanos end-to-end tests (8, 2)	4m 28s
Thanos end-to-end tests (8, 3)	5m 23s
Thanos end-to-end tests (8, 4)	8m 2s
Thanos end-to-end tests (8, 5)	4m 54s
Thanos end-to-end tests (8, 6)	5m 8s
Thanos end-to-end tests (8, 7)	5m 24s
	1h 11m 15s

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