[201811][Cherry-pick] [acl mirror action] Mirror session ref count fix at acl rule attachment

[bingwang-ms]

What I did
This PR is a cherry-pick of #1761 from master branch to 201811 branch.
I have to create another PR to pick up the code since it's not a cleanup cherry-pick. Some UT code issues wrere fixed as well.
This PR will fix #1891

Why I did it
Please see #1761

How I verified it
Verifyed by UT

pytest test_mirror.py -v                                   
===================================================================================================================== test session starts ======================================================================================================================
platform linux2 -- Python 2.7.15+, pytest-3.3.0, py-1.10.0, pluggy-0.6.0 -- /usr/bin/python2.7
cachedir: .cache
rootdir: /home/bingwang/workspace/sonic-swss/tests, inifile:
collected 6 items                                                                                                                                                                                                                                              

test_mirror.py::TestMirror::test_MirrorAddRemove PASSED                                                                                                                                                                                                  [ 16%]
test_mirror.py::TestMirror::test_MirrorToVlanAddRemove PASSED                                                                                                                                                                                            [ 33%]
test_mirror.py::TestMirror::test_MirrorToLagAddRemove PASSED                                                                                                                                                                                             [ 50%]
test_mirror.py::TestMirror::test_MirrorDestMoveVlan PASSED                                                                                                                                                                                               [ 66%]
test_mirror.py::TestMirror::test_MirrorDestMoveLag PASSED                                                                                                                                                                                                [ 83%]
test_mirror.py::TestMirror::test_AclBindMirror PASSED                                                                                                                                                                                                    [100%]

================================================================================================================== 6 passed in 158.96 seconds ==================================================================================================================

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