Remove x/net/context vendor by using std package

[sapk]

This PR update

• github.com/markbates/goth
• github.com/blevesearch/bleve
• golang.org/x/oauth2

To remove the unneeded vendor x/net/context since Go1.7.

This PR is still in WIP since it wait for an other PR on golang.org/x/oauth2. golang/oauth2#339

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[techknowlogick]

@sapk can you link to PR that is blocking this in golang.org/x/oauth2?

[sapk]

@techknowlogick golang/oauth2#339

[techknowlogick]

Thanks @sapk

[sapk]

There is still x/net/context in vendor since I found that x/net/context/ctxhttp is use at one (and only) point in x/oauth2. I will try to push the fix to oauth2 as it not really needed to use ctxhttp for it.
Still this PR could be merge as it still update deps that were using x/net/context and will ease the process for the next.

[lunny]

conflicted

[sapk]

@lunny I rebased on master. It should be good.