Improve thread-safety of cow_ptr

[mantid-builder]

Original Reporter: Russell Taylor

When vectors are shared between spectra and an algorithm is breaking that sharing, you can have a problem with the original vector being destroyed because the check for uniqueness in cow_ptr::access always fails if multiple threads are checking at the same time. I.e. the reference count goes from, e.g., 2 to zero if two threads go through simultaneously on their final pass.

[mantid-builder]

This issue was originally trac ticket 1353