Only reap txs that have been in the proposer’s mempool for a configurable amount of time.

[evan-forbes]

part of #1334

In an effort to increase the number of txs in proposals that are already suitably distributed throughout the network, proposer can only include txs that have been in their mempool some period of time. Optionally, since larger txs take significantly longer to distribute than smaller txs, this can also be based on how large the tx is.