Remove all in-code use of leadership "instance name" + constrain column to single value

[brandur]

With the addition of #301, and more specifically the schema-based
namespacing that it brings in around listen/notify, we're fully moving
towards a world where the recommendation for running multiple Rivers in
a single database is very definitive: isolate them by schema.

The elector's had a long-standing parameter called "instance name"
that's stored into the river_leader table, and which was prospectively
going to be used for River namespacing, but something we never made use
of it. In a world of schema isolation, each schema will have its own
river_leader table, and no kind of additional namespacing is needed
within the table.

I was originally going to try and approach herein we drop name out of
river_leader completely, but looking more closely, it's the table's
primary key. We could add a new column that'd act as a primary key
instead (e.g. imagine a boolean primary key column with a check
constraint that makes sure it's always true), but nothing we'd add would
be that much better. Instead, I elected to give name a default value
of default (matching the previous default instance name), and add a
check constraint verifying that it's always default, making it
effectively a single row table. The nice part about this approach is
that we can put these changes into the V4 migration in #301, and we
won't require any additional changes in any future migration.

With name now constrained to a single value, we can simplify all the
river_leader-based queries by removing their name parameters, then
remove instance name completely from elector code and drivers, giving us
a thorough overall cleanup.

[brandur]

@bgentry Thoughts on this approach? See PR description, but I realized that name was acting as the primary key for river_leader. We could change the key to something else, but it'd involve adding some new dummy column to act as one, so I opted instead to default name to default, and add a check constraint to verify that only the value default is allowed. An advantage of this compared to what you and I had discussed before is that name will stay around, meaning no future migration will be required.

[bgentry]

Works for me!

[brandur]

Wicked, thanks!