cluster_resolver: wait until the configs have been propagated to all the children

[aranjans]

What does this PR do?

This PR ensures that every time configuration update is triggered, it waits for child policy configuration update inline.

Approach:

Added a channel to ccUpdate of which that channel will be closed after child policies are updated. Same is verified in the test by having a ClientConnUpdateHook() which is called after the above mentioned channel is closed, which ensures child policy was updated synchronously.

RELEASE NOTES:

• cluster_resolver: wait for child policy configuration to be updated inline