Add stage/transfer API tests to CI framework

Description

This PR incorporates, finishes, and adds to the work done in PRs #596 and #644 Thanks to Craig for doing the majority of this.

Creates lower-level tests (t/ci/800-stage-tests.sh) to do a proof of concept of the transfer API. Also creates a higher-level file (t/ci/RUN_CI_STAGE_TESTS.sh) that runs the lower-level tests over a sweep of file sizes and server configurations.

Added a parallel transfer test to t/ci/800-stage-tests.sh as well. It is currently being skipped until the parallel transfer logic is fixed. Issue #686

Fixes a bug in serial transfer logic causing the destination file to be created as read-only, resulting in a failed transfer.

Update transfer docs to account for the various methods.

Motivation and Context

Add tests for UnifyFS transfer API

How Has This Been Tested?

Running the new 800-stage-tests.sh and RUN_CI_STAGE_TESTS.sh on one to four nodes with multiple processes per node.

Types of changes

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Performance enhancement (non-breaking change which improves efficiency)
[ ] Code cleanup (non-breaking change which makes code smaller or more readable)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[x] Testing (addition of new tests or update to current tests)
[x] Documentation (a change to man pages or other documentation)

Checklist:

[x] My code follows the UnifyFS code style requirements.
[x] I have updated the documentation accordingly.
[x] I have read the CONTRIBUTING document.
[x] I have added tests to cover my changes.
[x] All new and existing tests passed.
[x] All commit messages are properly formatted.

LLNL / UnifyFS