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 Won't this block always execute since `key == nil` when it is at the end of an object?