To prevent the need to copy the entire onion_error.go file for a new
Mission Control migration, this commit just updates the existing
lnwire21/onion_error.go file with the new CodeInvalidBlinding code. The
lnwire21 should not really ever be updated but adding a new code should
be fine as it does not affect old migrations since this is a new code.

In preparation for the commit which will add the main logic for
migration 32 (which will migrate the MC store to use a more minimal
encoding), this commit just adds some of the code that the migration
will need to the package.

Add a new mcRoute type that houses the data about a route that MC
actually uses. Then add a migration (channeldb/migration32) that
migrates the existing store from its current serialisation to the new,
more minimal serialisation.

So that `missionControlStore` can be unaware of the backing DB structure
it is writing to. In an upcoming commit when we change mission control
to write to namespaced buckets instead, we then only need to update the
`namespacedDB` implementation.

In this commit, the mission control store is migrated such that all
existing pairs which are currently stored directly in the top level
results bucket are now instead moved to a "default" namespace bucket.

Note that this migration is not yet invoked in this commit. The
migration will be invoked in the same commit that starts writing and
reading the new format.

and invoke the associated mission control migration.

Only the MissionControl instance should use this variable and it should
not be accessible to users of MissionControl.

This commit renames the previous MissionControl to MissionControlManager
since it will soon back multiple namespaced MissionControl instances.
For now, it just houses a single MissionControl in the default
namespace.