Fix initialization of `_device_list_id_gen` (#14914)

On startup, the `_device_list_id_gen` stream id generator is initialized
using the maximum stream id seen in a list of tables. When we started
populating the `device_list_remote_pending` table in #13913, we forgot
to add it to the aforementioned list of tables, so the stream id
generator can hand out old stream ids after a restart. The end result is
that Synapse can fail to handle device list update EDUs after a restart
when a partial state join is in progress.

Add the `device_list_remote_pending` table to the list of tables to
consider when initializing the `_device_list_id_gen` stream id generator.

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