add tx.FlushDBPages method

[lukechampine]

This method flushes pages from RES, reducing (perceived) memory scarcity.
I tested this by adding the following code to Sia's consensus set:

go func() {
	for range time.Tick(10 * time.Second) {
		cs.db.Update(func(tx *bolt.Tx) error {
			return tx.FlushDBPages()
		})
	}
}()

The result was, during rescan (specifically, rescanning a renter from height 0) memory usage steadily grew to about 1 GB RES, then dropped to about 200 MB RES once the rescan finished. Once this is merged, we can figure out how to best add intermittent flushing to the various Sia bolt dbs. For example, in the wallet and tpool where we already coalesce transactions with syncDB, we could also flush all pages there.