Allow control over parallel network activity

[sdboyer]

A bug was reported to glide (Masterminds/glide#411) about how concurrent git+ssh activity causes issues when the key hasn't been decrypted into an agent. To handle this case, vsolver must provide an option for fetching in serial so that the requests are made in serial, and each password input is recorded correctly.

In some imaginary, future version, we might also attempt to predetect which URLs are safe to parallelize (e.g., unsecure git http), and have this option (or a new one) always parallelize those, but not others.

Either that, or we've gotta do some gnarly shit with os.Stdin and os.Stdout across processes...

Affects #13

[fabulous-gopher]

This issue was moved to golang/dep#448