Do not send first blocks extension

[hannahhoward]

Goals

Implement #225

Implementation

• add the extension name an a utility to encode decode the data (just a number)
• add processing of the extension to the query preparer
• handle behavior for the extension in the response assembler. You can now specify a number of blocks (** non-unique **) on a request to traverse before they start going over the wire.
• add integration test to demonstrate flow

[rvagg]

nice, is it go-datatransfer that becomes responsible for using this?