Port configuration incremental update support

[Junchao-Mellanox]

Due to historical reason, portsyncd and portmgrd both handle PORT table changes in CONFIG_DB and write APPL_DB according to configuration change. portmgrd handles fields including "mtu", "admin_status" and "learn_mode"; portsyncd handles all fields. There are a few issues here:

1. portsyncd is designed to listen to kernel port status change and fire the change event to high level, it should not handle CONFIG_DB change. portmgrd is the right place to handle port configuration change according to SONiC architecture.
2. Configuration change for "mtu", "admin_status" and "learn_mode" will be handled twice which is not necessary
3. portsyncd put all configuration to APPL_DB even if user only changes part of them. E.g. user changes "speed" of the port, portsyncd will put configuration like "mtu", "admin_status", "autoneg" to APPL_DB. This is not necessary too.

This HLD describes how to fix them.

PR:

PR title	state	context
[sonic-swss] Port configuration incremental update support

[liat-grozovik]

@prgeor , @qiluo-msft could you please review and signoff? coding is ready and feature is required for 202205.