Open an old db is slow in V3.1 first time

Open a database from 3.0 or older, there is a big performance hit first time as Core converts it to V3.1 format.

In V3.1, we use a new table to store all deleted docs. As it reduces the size of the table for usual queries, it turns out very expensive the first time as we move all deleted docs to the new table. For a customer's db of 6.9G (CBSE-13748), it takes 90 seconds on Mac.