Protobuf for message payload.

Following are the numbers with goprotobuf,

  Encoded mutation length 135
  PASS

  BenchmarkCapMarshal          1000000              1129 ns/op
  BenchmarkCapUnmarshal        2000000               867 ns/op
  BenchmarkPoolMarshal         5000000               570 ns/op
  BenchmarkPoolUnmarshal       5000000               353 ns/op
  BenchmarkIndexInfoMarshal    1000000              1098 ns/op
  BenchmarkIndexInfoUnmarshal  5000000               480 ns/op
  BenchmarkMutationMarshal     1000000              1172 ns/op
  BenchmarkMutationUnmarshal   5000000               355 ns/op

Seems to take about 5-10% of additional overhead while encoding.
Updated README for using protobuf.