Add proposeRound action to witness.js contract

[AlexTrapp]

We propose that a pull request be created to modify the existing actions to the block signing workflow.

proposeRound action should be defined such that a block producing witness will "proposeRound" before the block is signed by all other witnesses. Information of this type might be stored in a "missedSign" or otherwised named variable.

If an apt witness proposes a new block, and the block fails to be produced due to some invalid signatures , the information in "missedSign" is more relevant than "missed block".

Regardless of storage of this data, by making a change to our p2p plugin routines to have witnesses broadcast the current signatures before the round ends, we can start using this action to submit partial signatures, and can ensure that in a crisis, apt witnesses stay up and participants with problematic connections are retired.

Thank you for your consideration,

LoColombia Block Producer Team