core.c: Change ssa member children counters update mechanism

Rather than updating ssa members children counters
when path records are received, they are updated in
'core_build_tree()' method and in case of unsuccessful
PR request are decremented.

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>