Add reference counting APIs to event subscription

[nanang]

Some application may process event subscription asynchonously, e.g: save pointer to evsub instance on evsub callback and return immediately from the callback while some other thread is assigned to continue some operation on the evsub instance. Unfortunately, the evsub instance may get destroyed before the other thread starts working on the evsub and later when it starts working on the evsub, the pointer to evsub is already invalid.

This ticket will introduce reference counting APIs so such application could safely keep a pointer to evsub instance.

Thanks George Joseph for the patch.

[nanang]

In 5351:

Close #1934: Add reference counting APIs to event subscription.