It doesn't actually work, and even if we could make it work reliably
we are better off if the records always know to go to the right shard.
It does make the interface a bit more complicated as we need to select
the right shard class with `for(account_id)`.
Instead we'll compute the table name dynamically based on
Current.account where needed. Also we'll prevent searchable records
from being saved if Current.account is not set, otherwise the after
commit callbacks will fail.
Donal McBreen