Fix memory leaks after import.

Overview:
When uploading a MARC file in an Iris environment, mod-data-import seems to use up all the memory necessary to process the file, but after the test is over, the memory does not get released. This causes the container to crash because it runs out of memory on subsequent imports. This situation is reproduced on the Vagrant Testing box too (see attachment).

Steps to Reproduce:
Upload a moderate file multiple times (500K records). Depending on the size of the file and the memory allocated to the container, the container will crash after 2 or more uploads. Can also observe the container's memory graphs on Cloudwatch or similar tools. 

Expected Results:
Memory is released after an import, multiple subsequent uploads of the same file will not crash the system.

Actual Results:
See above

Interested parties:
OleksiiKuzminov abreaux Kateryna Senchenko