[201911][everflow] Add retry mechanism for mirror sessions and policers

[bingwang-ms]

What I did
This PR is based on #1486. I have to open a PR for branch 201911 since it's not a clean cherry-pick.
This PR is to add retry mechanism for mirror session and policers for below scenarios

1. Attempting to create ACL rule when mirror session is not created or activated
2. Attempting to create mirror session when policer is not created.

Some UT issues were also addressed in this PR.

Why I did it
This PR is to add retry mechanism for mirror session and policers for below scenarios.

How I verified it

1. Verified by UT

test_warm_reboot.py::TestWarmReboot::test_PortSyncdWarmRestart PASSED                                                                                                                                                                                 [ 94%]
test_warm_reboot.py::TestWarmReboot::test_VlanMgrdWarmRestart PASSED                                                                                                                                                                                  [ 94%]
test_warm_reboot.py::TestWarmReboot::test_swss_neighbor_syncup  ^HPASSED                                                                                                                                                                                 [ 95%]
test_warm_reboot.py::TestWarmReboot::test_OrchagentWarmRestartReadyCheck PASSED                                                                                                                                                                       [ 95%]
test_warm_reboot.py::TestWarmReboot::test_swss_port_state_syncup PASSED                                                                                                                                                                               [ 96%]
test_warm_reboot.py::TestWarmReboot::test_routing_WarmRestart  ^H ^H ^HPASSED                                                                                                                                                                                  [ 96%]
test_warm_reboot.py::TestWarmReboot::test_system_warmreboot_neighbor_syncup  ^HPASSED                                                                                                                                                                    [ 97%]
test_warm_reboot.py::TestWarmReboot::test_VrfMgrdWarmRestart PASSED                                                                                                                                                                                   [ 97%]
test_warm_reboot.py::TestWarmReboot::test_MirrorSessionWarmReboot PASSED                                                                                                                                                                              [ 97%]
test_warm_reboot.py::TestWarmReboot::test_EverflowWarmReboot PASSED                                                                                                                                                                                   [ 98%]
test_warm_reboot.py::test_nonflaky_dummy PASSED                                                                                                                                                                                                       [ 98%]

2. Verified on a testbed by running warm-reboot with warm-reboot assistant. The mirror session and ACL rules were recovered after warm-reboot.

Details if related

[lgtm-com]

This pull request fixes 1 alert when merging 53748f6 into ce8b1af - view on LGTM.com

fixed alerts:

• 1 for Unused import

[prsunny]

@bingwang-ms , do we need this as a functionality for 201911 or is it a nice-to-have feature? 201911 is currently stable and we probably don't need enhancements but only bug fixes.