change "pool" directive parser to be more functional. Equivalent changes already in "tracing" branch

rename things, add query_threshold

make the original "latency" directive more versatile and alias it with the new preferred directive "query_latency"
use caddy.ParseDuration rather than time.ParseDuration

add TypeName() and Stringer implementation for each packet type
update PendingPacket to implement TypeName and fmt.Stringer

re-factoring things that have context as or in the name
move some type defs closer to code that uses them (deleting type file)