Support time with escrow

[alpe]

Is your feature request related to a problem? Please describe.

Allow use of header time rather than header height as a trigger for timeout. Since tendermint 0.27 the timestamp in the header should be considered "BFT time" (cannot be faked without collusion of majority of the nodes), and is a reliable and more predictable indicator than block height (especially when no-empty-blocks is enabled)

Copied from #310

[ethanfrey]

Please note that the time is taken from the block header.
This means we also need to set and read this value from the header in our app and the tests