Tweak the meaning of HTTP timeouts

Tweak the meaning of HTTP timeouts

Previously a timeout was set via libcurl's blanket timeout option, which is a
timeout for the entire request. This isn't always what we want, however, as
cargo is used on quite a variety of networks. Instead what we really want is
timing out data being received, so instead of a blanket timeout we set two
different timeouts:

* The connect timeout is now configured (time it takes to connect the socket)
* A "low speed" timeout is now also set. This means that if Cargo doesn't
  receive 10 bytes of data in the specified tiemout period that the entire
  transfer will be timed out.

Closes #1560