Readable cassette payloads for Thrift+VCR

[abhinav]

Recorded Thrift payloads should be readable (and editable by humans). thriftrw makes this simpler to do. We'll have to figure out a migration path for existing cassettes that are using base64 blobs.

[HelloGrayson]

Let's wait until we drop Apache Thrift support.

[abhinav]

Yeah. When we add this, it'll be to the mash release branch or after that's released.