core.c: Make core_start_dtree_timer() method more generic

core_start_dtree_timer() method was renamed to core_start_timer()

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
core.c: Add timer for calling core_rebalance_tree()

Currently core_rebalance_tree() is called only once:
CORE_BALANCE_TIMEOUT seconds after the first subnet up
event.

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