BGP: Check for duplicate and overlapping listen ranges

BGP: Check for duplicate and overlapping listen ranges

When configuring listen ranges for allowing dynamic BGP neighbors,
ensure that there are no duplicate or overlapping ones. This is
necessary because at the time of handling an incoming connection,
the first range that matches the source of the connection (and hence,
its peer-group parameters) will be used.

Signed-off-by: Vivek Venkataraman <vivek@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Atul Patel <atul@cumulusnetworks.com>
Ticket: CM-5153
Reviewed By: CCR-3714
Testing Done: Manual verification