[Link Event Damping] Add Abstract SelectEventHandler class to handle link event damping timer events.

[Ashish1805]

• Handler in the class will be used to handle the timer event of link event dampers.

HLD: sonic-net/SONiC#1071

[Ashish1805]

Adding @Junchao-Mellanox for review.

[Junchao-Mellanox]

do we really need this?

[kcudnik]

Why constructor is protected ?

[kcudnik]

Why constructor is protected ?

[Ashish1805]

@kcudnik Please let me know if you have any other comments/concerns. If everything looks good, can you please merge the PR? Thank you.

[kcudnik]

You did not answered my questions about protected constructors

[Ashish1805]

You did not answered my questions about protected constructors

In another comment reply, I had added the explanation:

Strictly speaking constructor is not needed, but constructor was added as protected just to make it clear to clients that they cannot directly instantiate an object of the class.

Agree, this class still be uninstantiable because of the pure virtual method, but to put the emphasize on not trying to directly instantiate this class, protected constructor was kept.

I am inclined to keep constructor as:
protected:
SelectableEventHandler() = default;

Thank you.