ssh/agent: sign data without a locked mutex

[Woellchen]

Currently signing requests are executed while the keyring mutex is
locked. However it is not necessary to do so. By unlocking as early
as possible we can take advantage of goroutines and serve different
signing requests in parallel.

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[gopherbot]

This PR (HEAD: 3732932) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/crypto/+/354611 to see it.

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See the Wiki page for more info

[gopherbot]

Message from Maxime Tremblay:

Patch Set 2: Code-Review+1

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Please don’t reply on this GitHub thread. Visit golang.org/cl/354611.
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