Introduced in: https://github.com/element-hq/synapse/pull/17167
The endpoint was part of experiments for MSC3575 but does not feature in
that MSC.
Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
If ther are more than 100 to-device messages pending for a device
`/sync` will only return the first 100, however the next batch token was
incorrectly calculated and so all other pending messages would be
dropped.
This is due to `txn.rowcount` only returning the number of rows that
*changed*, rather than the number *selected* in SQLite.
