crypto/tls: cleanup handshake state

[FiloSottile]

We should refactor where and when the hs and Conn state is accessed and modified during the handshake. For example checkForResumption should probably be side-effect free.

[SparrowLii]

I checked the checkForResumption() function and found that SessinState and hs.suite will be rewritten even if the function return false.

[gopherbot]

Change https://golang.org/cl/245160 mentions this issue: crypto/tls: make checkForResumption side-effect free

[gopherbot]

Change https://golang.org/cl/246038 mentions this issue: crypto/tls: simplify the process of cipher suites checking.

[gopherbot]

Change https://golang.org/cl/245837 mentions this issue: crypto/tls: supprtedVersions:return rightly for nil pointer parameter and plainer

[gopherbot]

Change https://golang.org/cl/246263 mentions this issue: crypto/tls: delete one useless judge statement.