In preparation to the Public Preview.

Missed this in 1b86618

Fix the following bug:

ManagedChannelImpl.ConfigSelectingClientCall may return early in start() leaving delegate null, and fails request() method after start().

Currently the bug can only be triggered when using xDS.

Fix a bug in StreamBufferingEncoder: when client receives GOWAY while there are pending streams due to MAX_CONCURRENT_STREAMS, we see the following error:
io.netty.handler.codec.http2.Http2Exception$StreamException: Maximum active streams violated for this endpoint.

Resolves #8097

pip 21.1 released on Apr 24 introduced a regression for python 3.6.1.
The regression was identified on Apr 24, the fix merged on Apr 25.
The fix is expected to be delivered in the 21.1.1 patch.

There's no clear date, when 21.1.1 will be released.
Until then, pin is temporarily pinned to the previous release, 21.0.1.

…8132) (#8155)

Triggering balancing state updates after already being shutdown can be confusing for the upstream of round_robin. In cases of the callers not managing round_robin's lifecycle (e.g., not ignoring updates after it shuts down round_robin, which it should), it can make problem very bad, especially with the behavior that round_robin is actually propagating TRANSIENT_FAILURE with a picker that buffers RPCs.

This change only polishes round_robin by always preserving its invariant. Callers/LBs should not rely on this and should still manage the balancing updates from its downstream correctly based on the downstream's lifetime.

…(v1.37 backport) (#8134) (#8156)

LoadBalancers should not propagate balancing state updates after itself being shutdown.

For LB policies that maintain a group of child LB policies with each having its independent lifetime, balancing state update propagations from each child LB policy can go out of the lifetime of its parent easily, especially for cases that balancing state update is put to the back of the queue and not propagated up inline.

For LBs that are simple pass-through in the middle of the LB tree structure, it isn't a big issue as its lifecycle would be the same as its child. Transitively, It would behave correctly as long as its downstream is doing in the right way.

This change is a sanity cleanup for LB policies that maintain multiple child LB policies to preserve the invariant that further balancing state updates from their child policies will not get propagated.

….x backport) (#8154) (#8160)

Inject a standalone Context that is independent of application RPCs to GrpclbLoadBalancer for control plane RPCs. The control plane RPC should be independent and not impacted by the lifetime of Context used for application RPCs.

…7.x backport) (#8161)

Control plane RPCs are independent of application RPCs, they can stand for completely different lifetime. So the context for making application RPCs should not be propagated to control plane RPCs.