Add a high priority proposal txs channel in CAT

[evan-forbes]

To distribute txs that in the proposal faster, we can add a separate channel in CAT that has a much higher priority than normal txs that are just being distributed in the mempool.

Optionally, as a [arguably premature] optimization, since these txs will be checked during process proposal, forcing them through CheckTx before enabling them to be gossiped isn't necessary. It should be noted that if they are not included in the proposal then they should be