Reuse grpc buffer in querier's store-gateway stream

[eeldaly]

What this PR does:
Reuse grpc buffer in querier's store-gateway stream to reduce memory usage in large queries

GRPC Store-Gateway Bench Test NEW:

BenchmarkGrpcStoreGatewayCalls-10    	    1501	    929363 ns/op	   59943 B/op	     455 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1723	    693808 ns/op	   49132 B/op	     456 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1818	    649024 ns/op	   49703 B/op	     454 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1516	    949340 ns/op	   46230 B/op	     453 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1765	    710793 ns/op	   50579 B/op	     453 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1670	    658193 ns/op	   50318 B/op	     458 allocs/op

GRPC Store-Gateway Bench Test OLD:

BenchmarkGrpcStoreGatewayCalls-10    	    1869	    717663 ns/op	  553930 B/op	     482 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1275	    849913 ns/op	  561863 B/op	     477 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1290	    834707 ns/op	  567643 B/op	     478 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1356	   1100831 ns/op	  560738 B/op	     477 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1146	   1179173 ns/op	  573945 B/op	     477 allocs/op
BenchmarkGrpcStoreGatewayCalls-10    	    1335	    875292 ns/op	  556640 B/op	     481 allocs/op

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]
• docs/configuration/v1-guarantees.md updated if this PR introduces experimental flags