Double buffer get and cat

We should double-buffer IPFS get and cat. That is, have one goroutine reading from the datastore and filling buffers to send back to the client and another goroutine actually writing these buffers.

From some basic testing with https://github.com/stebalien/go-dbuf, this gives a 10% speedup on an SSD with badger. However, I _think_ we can do better.