BUG/MINOR: dict: race condition fix when inserting dictionary entries.

BUG/MINOR: dict: race condition fix when inserting dictionary entries.

When checking the result of an ebis_insert() call in an ebtree with unique keys,
if already present, in place of freeing() the old one and return the new one,
rather the correct way is to free the new one, and return the old one. For
this, the __dict_insert() function was folded into dict_insert() as this
significantly simplifies the test of duplicates.

Thanks to Olivier for having reported this bug which came with this one:
"MINOR: dict: Add dictionary new data structure".