Don't validate the response in benchmark.

It's a memcached error by definition so I'm always going to get an
error, and then have to type assert it, etc...  That's all stuff that
isn't what I'm trying to measure here.
Client decode optimization.

BenchmarkDecodeResponse  587 ns/op  40.87 MB/s  174 B/op  4 allocs/op
BenchmarkDecodeResponse  400 ns/op  59.86 MB/s  166 B/op  3 allocs/op
Implemented MCResponse.Receive
Testing empty respnse parsing.
Testing empty request parsing.