Fix NettyChannelTracker race condition when tracking new channels

[injectives]

Cherry-pick of #904

This update fixes an issue when SimpleChannelPool may invoke the channelAcquired method before the channelCreated method is called. Since it violates NettyChannelTracker contract, it may result in runtime failures.

See the following for details:

@startuml
T1 -> SimpleChannelPool: acquireHealthyFromPoolOrNew
activate SimpleChannelPool

SimpleChannelPool -> NettyChannelPool: connectChannel
activate NettyChannelPool

NettyChannelPool -> ChannelFuture: addListener
activate ChannelFuture

ChannelFuture --> NettyChannelPool:
deactivate ChannelFuture

NettyChannelPool --> SimpleChannelPool:
deactivate NettyChannelPool

T2 -> ChannelFuture: setSuccess
activate ChannelFuture

ChannelFuture -> ChannelFuture: done
activate ChannelFuture

ChannelFuture --> ChannelFuture:
deactivate ChannelFuture

SimpleChannelPool -> ChannelFuture: isDone
activate ChannelFuture

ChannelFuture --> SimpleChannelPool:
deactivate ChannelFuture

alt isDone case
    SimpleChannelPool -> NettyChannelTracker: channelAcquired
    activate NettyChannelTracker

    NettyChannelTracker --> SimpleChannelPool:
    deactivate NettyChannelTracker
end

SimpleChannelPool --> T1:
deactivate SimpleChannelPool

ChannelFuture -> NettyChannelTracker: channelCreated
activate NettyChannelTracker

NettyChannelTracker -> ChannelFuture:
deactivate NettyChannelTracker

ChannelFuture -> T2:
deactivate ChannelFuture
@enduml

[michael-simons]

Thanks for back porting it. I double checked that the NettyChannelTracker is on the same state as well.