Refactor elector somewhat + test suite + poll-only mode

Here, do a little refactoring in the elector. The overall code doesn't
change that much, but we try to tighten things up with little things
like improved logging and real exponential backoff. It becomes a
start/stop service so that it's a little more normalized with other code
and more robust on start/stop.

The major improvement is the addition of a test suite. Although there
was a nominal suite before that was added during the driver refactor to
test the `attemptElectOrReelect` function, the elector was only tested
indirectly otherwise through the client. The change comes with a variety
of tests that exercise the elector's various behaviors, including one
that pits multiple competing electors against each other to make sure
that works.

Lastly, the elector gains a "poll only" mode in which we check that it
can still function using polling only in case a listener isn't
available. This doesn't have any effect on River feature-wise yet, but
the idea is that after we've added a similar capability to the producer,
we'll be able to support systems where `LISTEN`/`NOTIFY` aren't
available like PgBouncer in transaction mode, or possibly even MySQL or
SQLite in the future. We send the poll only mode through the same
barrage of tests that we require it to pass when using a database pool.

client.go

@@ -461,11 +461,8 @@ func NewClient[TTx any](driver riverdriver.Driver[TTx], config *Config) (*Client
 		instanceName := "default"
 
 		client.notifier = notifier.New(archetype, driver.GetListener(), client.monitor.SetNotifierStatus)
-		var err error
-		client.elector, err = leadership.NewElector(archetype, driver.GetExecutor(), client.notifier, instanceName, client.ID(), 5*time.Second, 10*time.Second, logger)
-		if err != nil {
-			return nil, err
-		}
+
+		client.elector = leadership.NewElector(archetype, driver.GetExecutor(), client.notifier, instanceName, client.ID())
 
 		if err := client.provisionProducers(); err != nil {
 			return nil, err
@@ -637,11 +634,9 @@ func (c *Client[TTx]) Start(ctx context.Context) error {
 			return err
 		}
 
-		c.wg.Add(1)
-		go func() {
-			c.elector.Run(fetchNewWorkCtx)
-			c.wg.Done()
-		}()
+		if err := c.elector.Start(fetchNewWorkCtx); err != nil {
+			return err
+		}
 	}
 
 	c.runProducers(fetchNewWorkCtx, workCtx)
@@ -664,6 +659,7 @@ func (c *Client[TTx]) signalStopComplete(ctx context.Context) {
 	// complete. We probably need a timeout or way to move on in those cases.
 	c.completer.Wait()
 
+	c.elector.Stop()
 	c.notifier.Stop()
 	c.queueMaintainer.Stop()