[core] [minor] Make sure ConnectionManager stops.

[vanzin]

My previous fix (force a selector wakeup) didn't seem to work since
I ran into the hang again. So change the code a bit to be more
explicit about the condition when the selector thread should exit.

[vanzin]

I ran the test that hanged a couple of times for me in a loop with this change, and didn't see any hangs. Still, with these things, it's hard to know.

[SparkQA]

Test build #30504 has finished for PR 5566 at commit ddb2c03.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.
• This patch does not change any dependencies.

[andrewor14]

For those following, this is a hot fix for #5335. @srowen

[srowen]

I see, did the call to wakeup() create a new problem? I thought it was just trying to solve a hang, but, that itdidn't do the trick. In any event seems fine as an addendum to SPARK-4194.

[vanzin]

No, the change in the previous commit didn't cause a problem (so technically this isn't a hotfix?), but it didn't solve the problem either. :-/