Reuse size, header, blob decode byte buffers
Reuse size, header, blob decode byte buffers

On my amd64 macbook pro with go1.7

benchmark                       old ns/op      new ns/op      delta
BenchmarkDecode-4               2644119879     1732578144     -34.47%
BenchmarkDecodeConcurrent-4     2772070000     1807731956     -34.79%

qedus/osmpbf