Remove lots of timer_state structs as they just ate memory and only a few was

Remove lots of timer_state structs as they just ate memory and only a few was
ever used.  Now a single state is kept for net, tty and disk events resp.
Also, call the randomness from disk_unbusy instead of biodone, as biodone
gets a lot of virtual events (from virtual filesystems etc), and as a bonus:
feed xfer time and size into the entropy pool too.