plugin/core.c: Fix distribution tree epoch update on change

Update dtree_epoch_cur in the case of successful return
from core_build_tree()

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>
core.c: In core_dump_tree, fix check of distrib_tree_level for no logging

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