The value from the received field can be used in the future for
subscription scheduling if this is provided. This is better than
the `orig_message_ts` field as `received` is assigned at the very
start of the pipeline when Sentry receives the event (as opposed to
when Snuba gets the event). Switching to this field means any
delays in ingestion will be properly accounted for when determining
the window on which to schedule subscriptions.

This PR also:
- deprecates the legacy decoder since we have fully switched over to the new format
- switches orig_message_ts from datetime to float. Converting between
the two in encode/decode is pointless, and it introduces the possibility of
timezone issues. Simpler to just keep it a unix timestamp everywhere.