core: Add SQLite backlog idx: bufferid, messageid

core: Add SQLite backlog idx: bufferid, messageid

Connecting to the core with a big sqlite storage (>500MB) can become
extremely slow or even time out.  Adding an index over the tuples
(bufferid, messageid) speeds up the connection significantly, because
at connection time a lot of queries over these two values are
processed.

Modified by @digitalcircuit from original commit to add a new schema
upgrade.

Closes GH-313 and GH-244.