Store record_time for all update kinds

Part of #74.

• Keep microsecond precision in timestamps written to the datastore.
• Store the Ledger API record_time of every transaction and reassignment in __transactions.record_time (V049, with a BRIN index for nearest_offset).
• Export it on spans as daml.record_time, and measure tx_lag_from_ledger_wallclock from it so reassignments count towards it.