mon: PaxosService: drop atomic_t on 'proposing'

We don't need this to be atomic -- a simple boolean is enough.

Fixes: #4507

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #315 from ceph/wip-4507

Reviewed-by: Sage Weil <sage@inktank.com>