Add timer for propose block

[orogvany]

Since we're moving to a gas driven block size We should be aware of low-performing validators.

If we're proposing a block, and we get down to 4 or so seconds, we should just propose what we have, instead of adding more transactions (or problematic slow transactions) that eat up our time.

Rather propose a block with less gas than max than miss our proposal window.

[orogvany]

This is probably unnecessary now that validators can specify their own gas limit, but this would help safeguard them all the same.