Optimize request signing

[dbemiller]

Minimizes new allocations inside the request signature. Consider this a work in progress... I'm just chipping away at it whenever I find myself waiting for someone.

This was based on Sweeney's advice that we use sync.Pool to ease garbage collector pressure. The primary goal is minimizing allocations per request... but that usually corresponds to faster speed improvements too.

Benchmark before changes:

Number of mallocs: 30

dbemiller-mac:azure dbemiller$ go test -bench=.
BenchmarkSignature-8   	  300000	      4275 ns/op
PASS
ok  	github.com/Prebid-org/prebid-cache/azure	1.345s
dbemiller-mac:azure dbemiller$ go test -bench=.
BenchmarkSignature-8   	  300000	      4247 ns/op
PASS
ok  	github.com/Prebid-org/prebid-cache/azure	1.335s
dbemiller-mac:azure dbemiller$ go test -bench=.
BenchmarkSignature-8   	  300000	      4366 ns/op
PASS
ok  	github.com/Prebid-org/prebid-cache/azure	1.373s

Benchmark after changes:

Number of mallocs: 19

dbemiller-mac:azure dbemiller$ go test -bench=.                      
BenchmarkSignature-8   	  500000	      2953 ns/op
PASS
ok  	github.com/Prebid-org/prebid-cache/azure	1.522s
dbemiller-mac:azure dbemiller$ go test -bench=.                    
BenchmarkSignature-8   	  500000	      2938 ns/op
PASS
ok  	github.com/Prebid-org/prebid-cache/azure	1.513s
dbemiller-mac:azure dbemiller$ go test -bench=.                       
BenchmarkSignature-8   	  500000	      2951 ns/op
PASS
ok  	github.com/Prebid-org/prebid-cache/azure	1.519s