To perform an emulated upsert into a table safely, we must either:
 * lock the table,
 * be the only writer upserting into the table
 * or rely on another unique index being present.

When the 2nd or 3rd cases were applicable, we previously avoided locking
the table as an optimization. However, as seen in #14406, it is easy to
slip up when adding new schema deltas and corrupt the database.

Since #13760, Synapse has required SQLite >= 3.27.0, which has support
for native upserts. This means that we now only perform emulated upserts
while waiting for background updates to add unique indexes.

Since emulated upserts are far less frequent now, let's remove the
option to skip locking tables, so that we don't shoot ourselves in the
foot again.

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

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