Make string caching in RLoopManager thread-safe

[vepadulano]

Implement and use a thread-safe container for caching strings that are created and used in the computation graph. Notably, these include the strings representing column names generated by Define.

This fixes the recent failures seen on MacOS in the dataframe-vary unittest. Since that underlines a thread-safety issue in the overall initialisation phase of MT RDF runs, it should be backported to 6.32

[github-actions[bot]]

Test Results

    12 files      12 suites   2d 20h 14m 53s :stopwatch:  2 642 tests  2 470 :white_check_mark: 0 :zzz: 172 :x: 29 977 runs  29 635 :white_check_mark: 0 :zzz: 342 :x:

For more details on these failures, see this check.

Results for commit 9233761c.