[rpc] modernize AcceptorPool's code

Since I'm modifying AcceptorPool implementation to add a new metric,
I went ahead and modernized the code a bit to logically separate
these mostly style-related updates from follow-up modifications.
This patch:
  * replaces Atomic32 field with std::atomic<bool>
  * updates the code to make it conform to the project style guide
  * other minor improvements

There are no functional modifications in this changelist.

Change-Id: I065510f846642098bc0182bc517324b88d175f0c
Reviewed-on: http://gerrit.cloudera.org:8080/20789
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
Tested-by: Abhishek Chennaka <achennaka@cloudera.com>
2 files changed