util: Use acceptResolvedAddresses() for MultiChildLb children

[ejona86]

A failing Status from acceptResolvedAddresses means something is wrong with the config, but parts of the config may still have been applied. Thus there are now two possible flows: errors that should prevent updateOverallBalancingState() and errors that should have no effect other than the return code. To manage that, MultChildLb must always be responsible for calling updateOverallBalancingState(). acceptResolvedAddressesInternal() was inlined to make that error processing easier. No existing usages actually needed to have logic between updating the children and regenerating the picker.

RingHashLb already was verifying that the address list was not empty, so the short-circuiting when acceptResolvedAddressesInternal() returned an error was impossible to trigger. WrrLb's updateWeightTask() calls the last picker, so it can run before acceptResolvedAddressesInternal(); the only part that matters is re-creating the weightUpdateTimer.

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When reviewing, you'll want to disable whitespace changes, especially for ring hash. There aren't that many changes, but most of this is more subtle than you might assume. I am quite happy to be able to remove other classes calling acceptResolvedAddressesInternal(), though.

[ejona86]

I'll merge this after the branch cut.