lib: Convert prefix_master->str to a RB Tree

lib: Convert prefix_master->str to a RB Tree

The prefix_master->str data structure was a sorted
list of the prefix names.  Not that big of a deal
other than insertion and deletion is insanely expensive
when you have a large number of unique prefix-lists.

In my test config file that I discovered this,
I have 587 unique prefix lists spread out acros
~26k lines of prefix-lists.  When reading
this config file into FRR the read time goes
from 690 seconds to 650 seconds.

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