Rough draft of merge, need queue of staging trees.

Rough draft of merge. Obviously, I need to put staging trees into a
queue. Instead of just two staging trees and toggling between them, a
queue of staging trees, adding new log entries to the head of the tree
and merging the remaining trees. This allows for a faster start. We can
add a new staging tree to the queue, merge any outstanding staging trees
at our leisure.

See #22.