Add support for automatic event expiration

I'm opening the PR early to get some direction in case I'm going off rails here. My current thinking is:
* simple global param `data-expiration` that will apply to all tables
* if the param is present start a background goroutine for each table that continuously walks through all the objects and sweeps them for expired events one by one
@mkobetic Yeah, a rollback is a lot faster. A commit will always write the meta page and the freelist. It'll probably also write the buckets that you accessed (even though they didn't change). I need to add dirty flags to that stuff so we can ignore it.