Refactor payload serializer

[at15]

Existing serializer only have one method WriteInt, which makes it thread safe, since the method use nothing from the object, but this introduce the problem when we want to write multiple series

A better implementation might be

WriteOpen() { buf.Write("[") }
WriteInt( .....
WriteDouble( ....
WriteClose() { buf.Write("]") }
Reader() // or maybe we can make it a reader closer
Reset()

• deal with comma in json array
  • one way is to use a flag to indicate if this is the first element in array
  • a better would be always add comma, and remove the last one, when finalize
  • might use truncate

[at15]

just leave it as it is for now

[at15]

And it brought trouble in #33 because Start does not call Reset and server won't complain for invalid JSON.