mkdir : store directory entries in a separate table

As of HDDS-2940, all the directories from the path prefix get created as entries in the key table. as per the namespace proposal attached to HDDS-2939, directory entries need to be stored in a separate "directory" table. Files will continue to be stored in the key table, which can be thought of as the "file" table.

The advantage of a separate directory table is to make directory lookup more efficient - the entire table would fit into memory for a typical file based dataset.