Adjust default block_proposal_timeout_ms

[obycode]

For the next release, we should adjust the signer config parameter block_proposal_timeout_ms one way or another.

If we are able to include the miner changes in #5729, then the default value for this parameter should be lower than 10 minutes.

If we do not include that miner change in the release, then block_proposal_timeout_ms should default to something very high, e.g. 4 hours, since there is no chance of the previous miner mining a tenure extend, so we should not reject a late valid block from the current miner.