ldpd: be more conservative with labels

ldpd: be more conservative with labels

On unstable networks, routes can be lost and relearned very often. If
we deallocate the input label every time a route is lost and allocate
a new one when the route is relearned, a lot of changes are made in vain.

This patch introduces a logic in which labels are preserved for at least
five minutes before being deallocated by the LIB garbage collector. This
is consistent with what other implementations do.

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