fix dht_bootstrap_alert being posted. add additional alert for dht put completion. add utility to test immutable put/get. fix issue in DHT preventing stores on router nodes (even when they return write tokens). immutable put and get confirmed to be working

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9743 a83610d8-ad2a-0410-a6ab-fc0612d85776
merged file priority fix from RC_0_16 and keep the disk thread's file priorities up to date

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9746 a83610d8-ad2a-0410-a6ab-fc0612d85776