Should we change the default blocking io dispatcher size?

[He-Pin]

Motivation:
The current default blocking io dispatcher is fixed size 16, I think it's too big.

Better with a non-fixed-sized one?eg default to 1, and size up to 16 if needed. But yes, user can adjust this themself.

[laglangyue]

I agree that it can be changed. And default value should be 16, because users can upgrade without being affected.