Do not accept investments to a crowdsale beginning when the deadline arrives, regardless of the blocktime of subsequent simple sends

[marv-engine]

Per J.R. in OmniLayer/spec#131

As long as the crowdsale is closed forever once it is closed, I think we are ok. We do need to communicate that a bitcoin block might close the crowdsale earlier than expected, but from a protocol perspective, we do NOT want a crowdsale to close and then reopen later because of an earlier timestamp. That would just be weird.

All our uses of the blockchain timestamp need to be one-time triggers since bitcoin time can temporarily move backwards.