Allow AttributeRegistry to signal treat-as-withdraw

Allow AttributeRegistry to signal treat-as-withdraw

When any attribute indicates we should be treating the resulting
message as withdraw, we need to report this indication upwards,
so that the it can be evaluated once the complete contents of
the message are known.

This can result in either a conversion of reachability to
unreachability, or, if we fail to parse the NLRI-bearing fields,
in a session teardown.

Change-Id: I8c3b7b9d9f9523b42a328820cc6b7c9168976aeb
JIRA: BGPCEP-359
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>