ldpd: schedule the sending of label messages when necessary

ldpd: schedule the sending of label messages when necessary

Once we send a Label Withdraw, we can't send a Label Mapping for the
same FEC until we receive a Label Release from the peer. This is due to
some limitations in the LDP algorithms described in Appendix A. ("LDP
Label Distribution Procedures") of RFC 5036.

To workaround this issue, make it possible to schedule the sending of
a Label Mapping as soon as a Label Release is received for the same FEC.

The easiest way to test this patch is by typing the "label local advertise
explicit-null" command. ldpd will withdraw all null labels using a
Wildcard FEC and then send new Label Mappings as soon the corresponding
Label Releases are received.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>