Add Context variants for Send and Receive

[kennyp]

Rationale

Closes #215 and fixes a double lock bug.

Note

A number of files were updated due to running go fmt. Happy to revert those and submit as a separate PR.

[gdamore]

You kept the gofmt a separate commit, so it's all good. :-) I've always done the go fmt thing, but its been a while and newer versions of go have probably changed things.

[gdamore]

I have mixed feelings about this ... not be cause I dislike using contexts, I think contexts are great -- mangos was designed before they existed is all.

My hesitation here is two fold:

1. The naming confusion -- I have a feeling that we should really rename mangos contexts to something else (I don't have a good answer) to avoid the confusion.
2. I almost think we should just make passing ctx mandatory, or perhaps make it an optional argument ...ctx.....

Let me think on this -- but think about the second point. I want to come to a solution here, but we might need to be willing to make a bigger change ... like another major version of mangos.

[kennyp]

I have mixed feelings about this ... not be cause I dislike using contexts, I think contexts are great -- mangos was designed before they existed is all.

My hesitation here is two fold:

1. The naming confusion -- I have a feeling that we should really rename mangos contexts to something else (I don't have a good answer) to avoid the confusion.

Yeah, I was really torn about that bit too. I did feel like allowing the stdlib to be called context was better than aliasing it to make it less confusing to new contributors.

2. I almost think we should just make passing ctx mandatory, or perhaps make it an optional argument ...ctx.....

Yeah, having context as required would seem to follow the Go convention for IO these days. I could see the argument for v4, but if we're going there what else would you want to include?

Let me think on this -- but think about the second point. I want to come to a solution here, but we might need to be willing to make a bigger change ... like another major version of mangos.