Change all FOPs to the same priority to reduce starvation issues @new-branch: feat_t2543483

Summary: Changed all the FOPs to the same (high) priority

Test Plan:
Test if this works on a simple setup, but need to check
on a more sophisticated setup

Reviewers: rwareing

Reviewed By: rwareing

CC: ethanr, cjh

Differential Revision: https://phabricator.fb.com/D862300

Task ID: 2543483

@new-branch: feat_t2543483
Perf fix for io-threads.

Summary: This diff slightly increases perf for larger io sizes, but we can't find a good reason why. We need to move this into a feature branch, create a build and do some testing on more real-world io.  This is a modified version of https://phabricator.fb.com/D860678  which was abandoned.

Test Plan: Test with I/O on a gluster cluster, perftools, and GDB

Reviewers: rwareing, ethanr

Reviewed By: rwareing

CC: cjh

Differential Revision: https://phabricator.fb.com/D862533