xds: Do not failover priority when IDLE->CONNECTING

[dapengzhang0]

Consists of two commits, the fix is in 2nd one, and the 1st only improves the test and does not impact the fix even without it.

[ejona86]

The first commit with the test changes looks fine. But the second one looks broken. It doesn't propagate the picker change up the tree, so the channel will continue using the old picker. READY→CONNECTING will thus return an old transport for potentially a long time.

[dapengzhang0]

The first commit with the test changes looks fine. But the second one looks broken. It doesn't propagate the picker change up the tree, so the channel will continue using the old picker. READY→CONNECTING will thus return an old transport for potentially a long time.

That does break. Redone the fix.

[ejona86]

I need to stare at this new version a bit longer, but I'll mention that I thought the earlier version would have worked just by adding a call to tryNextPriority. Since the subpolicy state wasn't changed, we'd expect the tryNextPolicy to be equivalent to the last time it was called, except it will pick up the new picker.

[dapengzhang0]

I thought the earlier version would have worked just by adding a call to tryNextPriority.

That would also work, but I really don't like to update the overall state like (READY, BUFFER_PICKER) in which READY is the previous child state and BUFFER_PICKER is the latest child picker