lib: Increase zapi buffer size

lib: Increase zapi buffer size

The buffer size is currently 4k.  Increase x4 times to allow for bigger
messages to be sent over the zapi.

The current size sufficient for most cases, but there are a couple
of cases with installing data to the kernel ip rules where we will
quickly hit this 4k size limit.  I forsee flowspec getting close
to this limit as well.

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