[sftp] put request chunk debugging into own category

Summary:
it's excessively verbose and not necessarily useful unless one is actually
debugging request handling, so separating this from the general slave
debugging seems handy as one can enable one without the other.

CCBUG: 368877

Test Plan: builds, installs, has separate logging categories

Reviewers: asn, broulik, ngraham

Reviewed By: broulik

Subscribers: kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D16387