Fix blocking I/O on main thread

These fix a number of issues that were causing I/O blockages on the main thread making the shell stall during heavy I/O workloads.

With them applied, I can use Shell on CentOS/RHEL 7 without stalls while doing a lot of background I/O (generated with a heavy fio script which exhausts ram with heavy sequential write load).

I used https://gitlab.gnome.org/chergert/iobt as a LD_PRELOAD to catch various fsync/open/etc syscalls and load the callgraph into Sysprof.

Just a reminder that this is for the 3.28, most patches are already available on master or have separate MRs