Tune reader_thread poll timeout value

[Frank Ch. Eigler]

Hi -

f.turgis wrote:

> [...]  1 last point to write and test patch as final as possible:
> Did you expect it also as a stap option or staprun option only [...]

I think it'd be OK to do it as a staprun-only option.  Over time I'd
like to automate the selection of this value rather than expose it
more broadly.

- FChE