Implement RFC 7313 enhanced route refresh. It is off by default and

Implement RFC 7313 enhanced route refresh. It is off by default and
can be enabled with 'announce enhanced refresh yes'
Similar to graceful restart this allows to mark routes as stale, refresh
them and the flush out routes that are still stale. Enhanced route refresh
uses a begin of rr and a end of rr message to signal the various stages.
A future enhancement would be the addition of a timeout in case the EoRR
message is not sent in reasonable time.
OK denis@ job@