Reuse the Context (DB connection) for requests.

It was impossible to apache-bench the system, but now it flies.
ab -n 1000 -c 10 http://localhost:8080/dspace-rest/collections/889
Before: 40/105 requests failed, mean = 3684ms, I interupted
After: 1000 requests success, mean = 9.6ms, (0.96ms with concurrent)