Switch keyd to allow multiple clients to be served at once

keyd was only accepting a single connection at a time and dealing with
all the requests from it before accepting the next client. This causes
delays in regular HKP processing while running something like wotsap on
a live keyserver.

Most of the pieces were already in place to deal with a per request select
based processing loop, so this commit switches to that model. A better
solution in the future may be to consider the use of libevent or libev,
but at present this enables the use of long runs stats programs while
still being able to service HKP requests in reasonable time.