Fix durable frame tracking to follow transaction boundaries (#2129)

Pull logic used invalid property of WAL in its implementation and was
written under assumption that any frames written to WAL are durable.

This is not true in case of frames written for unfinished transaction,
because such frames will be omitted by any SQLite connection except from
the one which created them. So, in case of restart - we will "loose"
these frames, but `durable_frame_num` will incorrectly state that we
already written "missed" portion of WAL.

This PR fix this issue and make pull to persist information about
"durable_frame_num" only at transaction boundaries.