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>
[core ssa].c: Add logging for mutex lock/unlock for debug

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