As part of the database migration to support threaded receipts, there is
a possible window in between
`73/08thread_receipts_non_null.sql.postgres` removing the original
unique constraints on `receipts_linearized` and `receipts_graph` and the
`reeipts_linearized_unique_index` and `receipts_graph_unique_index`
background updates from `72/08thread_receipts.sql` completing where
the unique constraints on `receipts_linearized` and `receipts_graph` are
missing. Any emulated upserts on these tables must therefore be
performed with a lock held, otherwise duplicate rows can end up in the
tables when there are concurrent emulated upserts. Fix the missing lock.

Note that emulated upserts no longer happen by default on sqlite, since
the minimum supported version of sqlite supports native upserts by
default now.

Finally, clean up any duplicate receipts that may have crept in before
trying to create the `receipts_graph_unique_index` and
`receipts_linearized_unique_index` unique indexes.

Signed-off-by: Sean Quah <seanq@matrix.org>