mutex savenexus calls to prevent segfaults

[walshmm]

Description of work

This adds a mutex for SaveNexus in MantidSnapper to mitigate the race condition we are experiencing

To test

NOTE: Test script points to data in MY directory, update paths before you run!

Attempt to reduce 64486, default calibration and artificial normalization is fine.
run cis script in pr at the same time.
Repeat till self assured.
Observe no segfault.

Link to EWM item

EWM#8534

Verification

• the author has read the EWM story and acceptance critera
• the reviewer has read the EWM story and acceptance criteria
• the reviewer certifies the acceptance criteria below reflect the criteria in EWM

Acceptance Criteria

This list is for ease of reference, and does not replace reading the EWM story as part of the review. Verify this list matches the EWM story before reviewing.

• acceptance criterion 1
• acceptance criterion 2

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.02%. Comparing base (c5e5379) to head (6b43602).
Report is 1 commits behind head on next.

Additional details and impacted files

@@           Coverage Diff           @@
##             next     #544   +/-   ##
=======================================
  Coverage   96.02%   96.02%           
=======================================
  Files          71       71           
  Lines        5463     5463           
=======================================
  Hits         5246     5246           
  Misses        217      217           

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[ekapadi]

I'm running the tests now, but you might as well fix the spelling error. That is the only comment that I had.

[ekapadi]

I verified that the provided cis-test script definitely produces a SEGFAULT on "next". (The script needs to be run in workbench at the same time as the reduction workflow is running in SNAPRed.) The SEGFAULT occurs during the final SaveNexus write of the reduced output workspaces.

On this branch however, the same test completes successfully without any SEGFAULT. I can definitely notice a slowdown in certain sections, usually where a RenameWorkspace operation is waiting on one of theSaveNexus calls. This behavior is reassuring as this indicates that the work-around is clearly being applied.