Events can grow stale when event_listen_queue is set.

[cro]

Description of Issue

When event_listen_queue on the master is set to large values (to reduce load and increase performance on busy infrastructures), in some cases there can be a lull in event traffic that will cause events in the listen queue to get stale. This creates the impression that jobs are taking much longer to complete than they should.

Steps to Reproduce Issue

Set event_listen_queue in /etc/salt/master to a large value (2500 for example). Restart the master. Wait for minions to reconnect. Run some commands (test.ping is sufficient). Note that it can take a long time for results to come back and/or the CLI may report minions as being disconnected.

Versions Report

Salt Version:
           Salt: 2018.3.0-1228-g07a3112

Dependency Versions:
           cffi: Not Installed
       cherrypy: Not Installed
       dateutil: Not Installed
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.10.1
        libgit2: Not Installed
        libnacl: 1.6.1
       M2Crypto: Not Installed
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.6.1
   mysql-python: Not Installed
      pycparser: Not Installed
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: Not Installed
         Python: 2.7.15rc1 (default, Nov 12 2018, 14:31:15)
   python-gnupg: Not Installed
         PyYAML: 3.13
          PyZMQ: 18.0.1
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.5.3
            ZMQ: 4.3.1

System Versions:
           dist: Ubuntu 18.04 bionic
         locale: UTF-8
        machine: x86_64
        release: 4.9.125-linuxkit
         system: Linux
        version: Ubuntu 18.04 bionic