For numbers with variable length, there should be a timeout to wait for
further digits before routing the number. This has been prepared with
the skinny-wait target, which waited forever. This patch implements the
digit timeout which routes the call after the timeout has elapsed. The
timeout can be configured in the mod_skinny XML settings
("digit-timeout") and defaults to 2 seconds.

This implementation has been requested and sponsored by Blackned GmbH.
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Nathan Neulinger <nneul@neulinger.org>