Use PostgreSQL instead of SQLite

I was very surprised how easy and fast and robust SQLite was, but in
the end I realized that its UPSERT support only came in version 3.24
and both Ubuntu 16.04 and 18.04 have older versions than that! I did
manage to install libsqlite3-0 from Ubuntu 18.04 cosmic on my xenial
host, but that feels dirty.

PostgreSQL has support for UPSERT since 9.5, not to mention the same
nice LIMIT and OFFSET clauses.

app.py

@@ -2,11 +2,12 @@
 # See DSpace Solr docs for tips about parameters
 # https://wiki.duraspace.org/display/DSPACE/Solr
 
-from database import database_connection_ro
+from database import database_connection
 import falcon
 from solr import solr_connection
 
-db = database_connection_ro()
+db = database_connection()
+db.set_session(readonly=True)
 solr = solr_connection()
 
 class AllItemsResource: