Allow selection of infinite metric routes

This fixes an issue where a selected route with infinite metric was
considered lost every time an update for that subnet came in,
potentially causing an update storm.

Signed-off-by: Lee Smet <lee.smet@hotmail.com>

src/source_table.rs

@@ -127,6 +127,7 @@ impl SourceTable {
             Some(fd) => {
                 (route.seqno().gt(&fd.seqno))
                     || (route.seqno() == fd.seqno && route.metric() < fd.metric)
+                    || route.metric().is_infinite()
             }
             None => true,
         }