#19 Improve accounting of used disk space

[Daniel Kunkle]

Currently, each node keeps track of how much data it *generates*, and merges local updates when that amount passes some threshold. What we really want is for each node to keep track of how much data is actually stored locally. This is somewhat complicated due to the use of remote writes. Probably, the best way to do this is to write a wrapper function for all methods that write or delete data on disk (e.g. Roomy_fwrite, Roomy_unlink, etc.). These wrappers can then keep track of used disk space.