Send file contents in chunks

Summary:
KIO can only send a maximum of 0xFFFFFF bytes of data.
TransferJob has an additional limit of 14 MiB.

BUG: 375765

Test Plan:
Downloaded a 132MiB file, no more "slave died unexpectedly".
Size and md5sum match.

Reviewers: elvisangelaccio, dfaure

Subscribers: dfaure, ngraham

Differential Revision: https://phabricator.kde.org/D15448