[signer] Improve DEFAULT_BLOCK_PROPOSAL_MAX_AGE_SECS

[jferrant]

It would be good to have a better default for DEFAULT_BLOCK_PROPOSAL_MAX_AGE_SECS that is some small multiple of the expected RTT for the peer network. Figure out what the current estimated RTT is go from there.

[jcnelson]

The consequences of leaving this as-is are that it's possible that the node will consider a stale block-proposal which will ultimately be rejected, but this is unlikely because there are other checks in the node that prevent stale messages from getting to this logic.