OPFS VFS: add the opfs-unlock-asap=1 URI flag which tells the VFS to release implicit...

OPFS VFS: add the opfs-unlock-asap=1 URI flag which tells the VFS to release implicit locks ASAP instead of during VFS idle time. This improves concurrency notably in the test app but brings a significant performance penalty in speedtest1 (roughly 4x slowdown). This is not the final word in OPFS concurrency, but gets us a step further.

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