Make work packet sizes easily configurable

[k-sareen]

You'd think that changing the work packet size here would be enough, but actually we need to change the size in VectorQueue as well as in the binding code if it has hardcoded the value. This needs to be fixed with a global constant.

See here for more discussion

[wks]

We can put all the constants in a dedicated module so that they can be easily looked up.

We may also allow the actual work packet sizes to be dynamically set at run time, overriding the defaults provided by constants.

[k-sareen]

Let's just do the former to fix it in the short-term and we can deliberate (on a separate issue) on whether we want to have the latter.