Fix EffectiveRibInWriter overwriting the entire route table

Fix EffectiveRibInWriter overwriting the entire route table

The fall-through in SUBTREE_MODIFIED case means that for any new write we
replace the entire route table, which has the effect of LocRib having to
run route selection even on routes which have not changed.

Fix this by removing the fall-through and deal with individual routes as appropriate.

Change-Id: I1297afc878d3d8bf9d0c0f461244cc4414f1caa1
Signed-off-by: Dana Kutenicsova <dkutenic@cisco.com>
Signed-off-by: Iveta Halanova <iveta.halanova@pantheon.sk>
(cherry picked from commit 1d6e02186ac85eff5734db59d3cbf5017214ddce)