collatejson optimization: tailored output for collatejson.Decode()

Sarath Lakshmanan suggested the following optimization,

• codec.Decode() returns JSON output, for couchbase 2i project
  the JSON string will always the following JSON format.
  [expr1, docid] - for simple key
  [expr1, expr2, ..., docid] - for composite key
  it would be a good optimization to implement a variant of Decode()
  that will return as,
  [expr1], docid - for simple key
  [expr1, expr2 ...], docid - for composite key

In scan-coordinator, this will help avoid repeated Marshal() and Unmarshal() on fdb results.