Add timestamp to the wire format and enforce timestamp check

[nik-suri]

Most EVM governance tokens calculate voting weight based on timestamp snapshots. Therefore, someone could leverage timestamp drift between a source and destination chain to double vote.

This change would add the source chain timestamp to the wire format, and enforce the following rule on the destination chain:

assert(dest_chain_timestamp > source_chain_timestamp)

It would therefore only be possible to complete transfers when the destination chain timestamp is greater than the source chain timestamp.