Memory leak (HttpClient)

Update:  Approved as R1 2021 Hot Fix at Capacity Planning Team meeting on May 17, 2021. 

Overview:

HTTP clients created within PubsubClient in PubSubClientUtils are not closed properly, resulting in a memory leak. See CIRC-1121 for details.

Steps to Reproduce:

1. Repeatedly run check-in and/or check-out process in mod-circulation.
2. Take a memory dump of mod-circulation's process.
3. See that HttpClientImpl objects are piling up.

Expected Results:

HttpClientImpl objects are closed after use and picked up by garbage collector.

Actual Results:

HttpClientImpl objects are piling up in memory.