Build reminder updates with correct SCID field

Build reminder updates with correct SCID field

When the reminder updates were added, a dummy `ChannelUpdate` with
a number of zero'd fields were created under the assumption that
the zero'd fields would be ignored downstream when building
serialized updates. However, the SCID field was `assert`'ed on (and
serialized in the update), causing any reminder updates to cause an
assertion panic.

Instead, we do it the Right Way (tm) here and move the
only-sometimes-available fields into the update type enum, ensuring
we can't access "poison" fields downstream.