Standardize log messages from LB policies

When I was debugging one of the interop test failures where the LB policy tree on the channel looked something like this:

cluster_manager --> rls --> cds --> cluster_resolver --> priority --> cluster_impl --> ring_hash

The above is only part of the tree.

And all these LB policies throw logs for events like creation, new state from above, creating a child policy, new state from the child policy etc. But the format of these logs are different in subtle ways and it makes the whole process of debugging painful.

It would be nice to standardize the format and the events which every LB policy should be logging (at minimum). We are going to be running into issues where we are going to have to look at logs with 10+ LB policies and doing this asap is going to help us a great deal.