Investigate performance improvements

[masaeedu]

Performance isn't great. Things to try:

• Benchmark where things are now
• There's a number of issues on the SSH.NET repo about SFTP performance, see if can help out there
• Investigate local caching and preemptive loading in the background
• Try out Microsoft's openssh implementation over P/Invoke?
• Make logging optional

[Schroedi]

I found that disabling Dokan logging helps quite a bit.
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