Detect beginning of static channel status streaks.

Detect beginning of static channel status streaks.

Previously, whenever a channel hasn't received updates in
6+ days, we would automatically send incremental reminders
that only contain the update flags.

However, if a channel has been received updates
within that six-day-timeframe, but all those updates were
identical, it would result in no updates being added to the
snapshot because the mutation set ended up empty and that
data would get purged from the serialization.

In order to avoid the reminder logic being duped by
channels simply being consistent, we now look up the
beginning of the latest continuous stretch of non-mutating
channel updates. If a channel's details have not been
altered in more than six days, we now send reminders no
matter the frequency with which channel updates have been
received since.