[MINOR] global: add "tune.chksize" to change the default check buffer size

[MINOR] global: add "tune.chksize" to change the default check buffer size

HTTP content-based health checks will be involved in searching text in pages.
Some pages may not fit in the default buffer (16kB) and sometimes it might be
desired to have larger buffers in order to find patterns. Running checks on
smaller URIs is always preferred of course.
(cherry picked from commit 043f44aeb835f3d0b57626c4276581a73600b6b1)