QBFT RLP for interop

[jframe]

Update the QBFT proposal RLP encoding and decoding for the new proposed interop format.

The proposal RLP encoding is defined as:

RLP([[[sequence, round, block], signature], [signedRoundChangeMessages, signedPrepareMessages]])

signedRoundChangeMessages = list of [[height, round, preparedValue, preparedRound], signature]

signedPrepareMessages = list of [[sequence, round, digest], signature]