[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