bfdd: avoid creating duplicate peer contexts

bfdd: avoid creating duplicate peer contexts

when configuring bfd peers, some parameters may or may not be taken into
account to search for a previous context. This has as consequence that
the result is different with the order of vty commands:

bfd
peer 4.5.6.7 vrf AAAA local-address 1.2.3.5
end
peer 4.5.6.7 vrf AAAA   <--- should not create new session
end

Similarly, the user thinks it has overwritten some key parameters like
local address, whereas it is wrong.
here, some informational message should be present.

bfd
peer 4.5.6.7 vrf AAAA
end
peer 4.5.6.7 vrf AAAA local-address 1.2.3.5
<--- should inform that the key lookup 1.2.3.5 is wrong
end

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>