piv: permit use of shared smartcard connections

[tianon]

• Closes piv: permit use of shared smartcard connections #108 (rebase/carry)
• Closes adding Shared access to multiple processes #100

Quoting #108 (comment):

I needed this, so I took the liberty of rebasing and updating it (which was a little non-trivial, especially in piv_test.go), and I think I've got the same spirit as all the changes here (many of which I had to reimplement green). 🙇

I've done some testing with it and it resolved a lot of issues I was having (I'm implementing something that can do ECDSA signatures via a YubiKey against this library, and before applying this, any concurrent attempts to sign would fail, and with this change I can happily make seemingly as many concurrent requests as I want and they just stack up, which is exactly how I want it 💖).

FWIW, I've been testing on a YubiKey 5C, although I will admit I have not run those updated tests (only verified that they compile correctly).

I've now been testing this very actively in a project that does a lot of concurrent accesses to a single YubiKey (via separate processes) to do ECDSA signing and it's been working like a champion for an entire month straight. 😄

Of course, I'm happy to rebase, amend, close, etc as desired! The branch has been out there linked from that PR for a bit but I figured maybe it'll be easier to look at/review if I explicitly opened it as a real PR. ❤️

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• Updates See if cached PIN signing is possible without requiring a PS/SC transaction #47
• Closes (maybe?) Windows - exclusive transaction is only usable for 5 seconds #172
• Closes Error: Unable to open the Yubikey card: connecting to smart card: the smart card cannot be accessed because of other connections outstanding #116