crypto/tls: avoid creating handshake context on each read/write

[moredure]

handshakeCtx and cancel function always allocated on read and write operation not depending on whether the handshake is completed

[gopherbot]

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[dt]

I believe there is a similar CL here: https://golang.org/cl/379034

[moredure]

@dt, I guess so, but I just moved the part of handshake completion/error check to the top, instead of checking it's completion in two places in a single Handshake call.
Also my PR kind of preventing simultaneous contexts/goroutines to be allocated in case of Two concurrent handshakes, so it's kind of more safe I guess.
Anyway I want to get rid of allocations ASAP) without breaking anything, that's why I made this pull request, maybe to show that's a problem and not only for a single person. Will be cool if it will be backported.
I also have a solution to get rid of context allocation itself in case of it won't be used in Handshake process by callbacks functions like GetClientCertificate, etc, but first things first.