Allocate memory from a file-backed pool

[geofft@mit.edu]

On most Athena shared machines, there's a lot more disk space available in each user's networked storage than for swap on the local machine. So, it makes sense that large barnowls should preferentially swap out data to that space, if possible. This is mostly doable if we modify our allocations for the data that tends to be persistent (in particular, owl_messages) to be in a pool that we swap to/from the user's homedir ourselves. Possibly easier than doing this at the memory level is to modify the objects' accessor functions to indirect through the disk, and just store offsets into the data file on disk in memory.