[portstat] Update portstat to use CounterTable API

[jimmyzhai]

What I did

To support gearbox port counter in CLI, following sonic-net/sonic-swss#2218 and sonic-net/sonic-swss-common#622.

How I did it

Use swsscommon CounterTable API and PortCounter type, covering different underlying switch architecture, asic-only, asic+gearbox.

How to verify it

junhuazhai@3221a2294104:/sonic/src/sonic-utilities$ pytest-3 tests/portstat_test.py
================================================================================================================================= test session starts =================================================================================================================================
platform linux -- Python 3.7.3, pytest-3.10.1, py-1.7.0, pluggy-0.8.0
rootdir: /sonic/src/sonic-utilities/tests, inifile: pytest.ini
plugins: pyfakefs-4.5.6, cov-2.6.0
collected 20 items

tests/portstat_test.py ....................                                                                                                                                                                                                                                     [100%]

============================================================================================================================= 20 passed in 40.83 seconds ==============================================================================================================================

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[jimmyzhai]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jimmyzhai]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).