Keep mirroring order books that hold orders which never expire

[droplister]

Counterparty Core now returns open orders with expiration 0 and
expire_index null. orders.expire_index was NOT NULL, so one such order
failed its market's whole write and sync_orders errored on every run,
leaving those books stale.

Migration 0040 rebuilds orders with a nullable expire_index, copying
every row and recreating both indexes. The indexer stores null for an
order that never expires (and still dates an expiring order Core sends
without expire_index), the read route passes it through, and the
activity, asset and profile order lists show "No expiry" instead of a
countdown or "GTC". The label is machine-translated in every locale and
listed as such in each status file.