[ycabled] move swsscommon API's from subroutines to call them exactly…

… once per task_worker/thread (sonic-net#303)

This PR attempts for ycabled to have all
swsscommon.Table and daemon_base.db_connect
as just a single call in the thread instance for all task_workers

For example all swsscommon calls to open Tables are moved as class object varables, which are reused when needed instead of opening the Table again in subroutines.

self.config_db[asic_id] = daemon_base.db_connect("CONFIG_DB", namespace)
This would help in avoiding unforeseen redis-errors
Signed-off-by: vaibhav-dahiya vdahiya@microsoft.com

Description
Motivation and Context
How Has This Been Tested?
Deploying changes on testbed and UT