bgpd: Speedup vtysh handling of 'show bgp afi safi json' display

bgpd: Speedup vtysh handling of 'show bgp afi safi json' display

When bgp has a metric butt-load of routes w/ ecmp, this command
can take an inordinate amount of time to run and complete via
vtysh.

Converting the bgp route output in this case back to not
using the json pretty-print drops ~2 minutes of runtime
off.

It is assumed that if users would like pretty output they
can run it through an appropriate tool via a pipe command.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>