Watch dir threads

[ChrisJohnNOAA]

Description

Adds a new approach to watch directories for changes. This utilizes one thread per file system instead of one thread per dataset being watched. This will drastically reduce the number of threads needed for watching datasets. The new approach does mean file change events are handled for all datasets, so updates will happen more frequently (matching the most updated dataset under the old approach, or even faster).

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• [X ] Bug fix (non-breaking change which fixes an issue)
• [X ] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist before requesting a review

• [X ] I have performed a self-review of my code
• [X ] My code follows the style guidelines of this project
• [X ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [X ] My changes generate no new warnings
• [ X] I have added tests that prove my fix is effective or that my feature works
• [ X] New and existing unit tests pass locally with my changes

[ChrisJohnNOAA]

Documentation updates are currently just in the 2.24 branch of the github io documentation https://github.com/ERDDAP/erddap.github.io/commit/6482ddd09d31e8e5f9ecb59d2fd5f599bc8ccde7