Refactor database code to use a context manager

Instead of opening one global persistent database connection when
the application I am now abstracting it to a class that I can use
in combination with Python's "with" context. Both connections and
cursors are kept for the context of each "with" block and closed
automatically when exiting.

See: https://alysivji.github.io/managing-resources-with-context-managers-pythonic.html
See: http://initd.org/psycopg/docs/connection.html#connection.close

dspace_statistics_api/database.py

@@ -7,9 +7,17 @@ import psycopg2
 import psycopg2.extras
 
 
-def database_connection():
-    connection = psycopg2.connect("dbname={} user={} password={} host={} port={}".format(DATABASE_NAME, DATABASE_USER, DATABASE_PASS, DATABASE_HOST, DATABASE_PORT), cursor_factory=psycopg2.extras.DictCursor)
+class DatabaseManager():
+    '''Manage database connection.'''
 
-    return connection
+    def __init__(self):
+        self._connection_uri = 'dbname={} user={} password={} host={} port={}'.format(DATABASE_NAME, DATABASE_USER, DATABASE_PASS, DATABASE_HOST, DATABASE_PORT)
+
+    def __enter__(self):
+        self._connection = psycopg2.connect(self._connection_uri, cursor_factory=psycopg2.extras.DictCursor)
+        return self._connection
+
+    def __exit__(self, exc_type, exc_value, exc_traceback):
+        self._connection.close()
 
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