[hostcfgd] Fix issue: FeatureHandler might override user configuration

[Junchao-Mellanox]

Why I did this?

hostcfgd is using a subscribe/listen way to handle configuration data. It handles feature configuration like this:

1. subscribe to CONFIG DB FEATURE table
2. read initial data from CONFIG DB FEATURE table and call FeatureHandler.sync_state_field to process the initial data
3. As FEATURE table "state" field might be a template, FeatureHandler is responsible for rendering it
4. FeatureHandler will "resync" the rendered "state" back to CONFIG DB

However, if there is a user configuration occurs between step 2 and step 3, user configuration will be silently ignored and override. Here is an example:

1. subscribe to CONFIG DB FEATURE table
2. read initial FEATURE data, say sflow.state = disabled. FeatureHandler.sync_state_field get an input that "sflow.state = disabled"
3. user issued command "config feature state sflow enabled". Now sflow.state is enabled in FEATURE table.
4. FeatureHandler.sync_state_field continues running and resync "sflow.state = disabled" back to FEATURE table

In this case, user configuration in step#3 will be override. The PR is a fix for this issue.

How I verified?

Added new unit test

----------- coverage: platform linux, python 3.9.2-final-0 -----------
Name                             Stmts   Miss  Cover
----------------------------------------------------
host_modules/config_engine.py       35      1    97%
host_modules/gcu.py                 54      0   100%
host_modules/host_service.py        19      3    84%
host_modules/showtech.py            27      7    74%
scripts/caclmgrd                   608    211    65%
scripts/determine-reboot-cause     144     13    91%
scripts/hostcfgd                  1236    261    79%
scripts/procdockerstatsd           157     90    43%
----------------------------------------------------
TOTAL                             2280    586    74%
Coverage HTML written to dir htmlcov
Coverage XML written to file coverage.xml

======================= 104 passed, 4 warnings in 6.44s ========================

[Junchao-Mellanox]

Hi @ganglyu , would you please kindly review?

[ganglyu]

LGTM

[Junchao-Mellanox]

Hi @ganglyu , could you please help merge this?

[Junchao-Mellanox]

It causes UT failure after cherry-picking to 202211, so I created a backport PR for 202211: #59