eth/protocols/snap: speed up hash checks

[holiman]

This PR optimises the hashing a bit during the snap response verification. We use a hash only temporarily, to verify against the request, and can thus reuse the same buffer, and avoid an alloc and copy. We can also use the keccatState to avoid a second alloc+copy via Read instead of Sum.

Benchmark for checking 10K items with 100 bytes of data each:

BenchmarkHashing/old
BenchmarkHashing/old-6         	     100	  20599259 ns/op	 5120510 B/op	   30002 allocs/op
BenchmarkHashing/new
BenchmarkHashing/new-6         	     219	   5355205 ns/op	     480 B/op	       2 allocs/op

[karalabe]

hash := make([]byte, 32), Go usually either uses var for the default value or := for non defaults. Pls also add a newline after this.

Pls update the others too.

[holiman]

Squashfixed

[karalabe]

SGTM

[DGKSK8LIFE]

formatting change; lgtm