sftp: break large writes into multiple requests

Summary:
servers have arbitrary limits that we should stay below. to ensure this
happens use MAX_XFER_BUF_SIZE as maximum size per request. if we read
data large than that, break it apart into multiple requests

(I am mostly guessing here, the rfc doesn't seem to impose any size
 constraint on the write requests themselves, so probably packet
 constraints apply by default)

BUG: 404890
FIXED-IN: 20.04.2

Test Plan:
- fallocate -l 128M file
- sftp to localhost
- copy file from one dir to another
- compare checksums match

- open xls file (via kio-fuse)
- change
- save
- close
- open again
- changes are still there

Reviewers: ngraham, meven, feverfew

Reviewed By: ngraham, meven, feverfew

Subscribers: meven, feverfew, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D29634