cbbackup-incremental synchronously stores data from the tap stream

The mbbackup tool is completely synchronous, so it's either receiving data from the network or writing to disk. I believe both of these things should happen concurrently.

Attached is a patch that runs all of the actual persistence through a separate thread (can easily be modified to use subprocess, though I don't think it makes a huge difference since most of the life of that thing will be in SQLite).