Specify PBBT and Compact Blocks

[cmwaters]

Ref: #1532

As the prototype shows more promising results and the design becomes almost finalized, we should then take the prototype and write up a comprehensive specification of the protocol such that it can be used as a reference for the full implementation and for audits. This applies to both Pull Based Broadcast Tree (PBBT) and Compact Blocks which can be done separately or together.