Add mutex to guard context tags

[ypatia]

For C-API tracing, we ended up using tags instead of config to store trace context for Jaeger traces, because we needed that to be mutable (to be able to update the tag when we'd like traces to nest). However, tags were not protected via some locking mechanism, so this PR is adding a mutex to guard tags_ variable.

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TYPE: NO_HISTORY DESC: Add mutex to guard context tags

[ypatia]

This facility has been moved entirely to class RestClient in #4994. I am currently working out the build problems there (the C++ code itself is done).

In addition, mutex protection needs to happen on read in addition to write. That's not currently possible with the way this facility is implemented with StorageManager in the middle.

Thank you @eric-hughes-tiledb for the heads up on those parallel changes. I checked your PR and added a comment.

[KiterLuc]

This will get closed once #4994 gets merged.

[KiterLuc]

After the REST refactor in #4994 this might be irrelevant or needs to be reimplemented. Closing for now.