Get Thrift IDL from Meta::thriftIDL

[kriskowal]

This allows TCurl to automatically obtain a ThriftIDL file from the service if one was not provided.

[prashantv]

Before doing this, I think we should update our libraries to use the newer thriftIDL response:
https://github.com/uber/tchannel/blob/master/thrift/meta.thrift

Since it's a backwards incompatible change, I feel like we should make the change before we start using it in production.

[kriskowal]

@prashantv Roger. Based on this work, it would be convenient to consolidate that entire shape into a single endpoint, both entry and map of sources. We appear to have elected to not burden static analysis and fetch-by-location on the client, with all that implies in flexibility/latency, so we may as well go all the way.

[kriskowal]

I am gifting this branch to @malandrew to carry it over the line. This will require the implementation of Meta::thriftIDL properly in tchannel-{node,python?,go?,java??}

[andrewdeandrade]

This will land as part of the completion branch. I'm going to look into checking if the different language versions of tchannel all support this endpoint since that needs to happen before this feature can be useful.

[kriskowal]

Fixes #10