internal/balancergroup: eliminate race in exitIdle

[easwars]

incomingMu needs to be taken before accessing scToSubBalancer map as part of exitIdle

RELEASE NOTES: n/a

[easwars]

I couldn't come up with a unit test for this. Please let me know if you have any ideas. Thanks.

[menghanl]

Make two goroutines with loops, one calls UpdateSubConnState, the other calls ExitIdle?
Without incoming mu, the two goroutines should race.

[dfawley]

So the balancer group calls into the wrapper calls back into the group?

Would it be better to put all this logic in the group's ExitIdle function, then? Or return a bool and have ExitIdle do the connect work?

[easwars]

Done.