pass GeoJSON in stringified form

[mourner]

This is a reapplication of #2001 without changing the API, and serves as the middle ground for improving setData performance as discussed in #1504.

cc @lucaswoj @jfirebaugh @dcervelli

[jfirebaugh]

Given http://nolanlawson.com/2016/02/29/high-performance-web-worker-messages/ I wonder if we should just build JSON.stringify / JSON.parse into Actor.

[mourner]

@jfirebaugh I thought about that, but it's tricky because of transferables — you don't want to stringify an array buffer.

[jfirebaugh]

Yeah, it would have to be only if the buffers argument to send is undefined.

[lucaswoj]

I'm 👍 on shipping this as-is and continuing to optimize per #1504