vfs.OpenFile is not thread safe

[puellanivis]

vfs.OpenFile contains many mutations of the underlying pbFS that are not performed under lock. Of particular note, the mutation of fs.data during file creation although there are also numerous modifications made to the underlying sealedFile that also are done without holding a mutex.

This could lead to unpredictable behavior in multithreaded contexts (which is includes any and all HTTP services) as two data file creations could race against each other potentially returning an alias of the other’s underlying sealedFile.