Add MemFS context isolation test, and use memfs in quickstart dense example.

[bekadavis9]

Through debug, we've discovered that the MemFS was not getting initialized in the VFS, despite being "enabled" by default. A global Context and VFS are typically used on the object so this bug was previously overlooked. This work item initializes the MemFS in the VFS and adds an example test, which also highlights the need for a global Context on MemFS.

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TYPE: BUG
DESC: Initialize MemFS in the VFS

[bekadavis9]

[sc-19989]

[shortcut-integration]

This pull request has been linked to Shortcut Story #19989: memfs initialization and testing.

[eddelbuettel]

Works great from R where we have a single Context object by default / design (allowing more or others as needed, but typically users just want one) and for that the (current ?) memfs setup works just fine.

[ypatia]

Thanks for the fix, good catch! I am worried about introducing a 2-step initialization and shared pointers, could you please check if we can avoid that before merging?

[ypatia]

Thank you for the changes! Let's wait for Isiah's/Eric's opinion on the ownership issue before we can proceed.
Also could you please add tests for your changes, at least a regression test for the bug you are fixing? I think it will be almost identical to your example, but I am not sure examples are run on CI so a test would make sense here.

[shaunrd0]

Was able to reproduce VFS.RemoveDir() errors using C# API and after building / linking with this branch errors were resolved. Thanks, this was good to review - lots of good information here

[eddelbuettel]

This is a bit confusing:

edd@rob:~/git/tiledb/examples/cpp_api(rd/memfs_error_debug)$ ./memfs 
Error: Must use process global Context on memfs.
2 3 4 6 7 8 
The MemFilesystem is associated with a single VFS instance.
edd@rob:~/git/tiledb/examples/cpp_api(rd/memfs_error_debug)$ 

So it throws on the attempt to vfs.remove_dir() the one memfs that there is, in the one context. If we disallow that, why not simply error on entry in the remove_array() (demo) function?

No functional changes remaining in this PR.