Removing WCNG-to-WCNG upgrade code for formats 12 to 19

We have code that auto-upgrades WCNG working copies. This is distinct
from the 1.6 to WCNG upgrade that is achieved using the "svn upgrade"
command. Format 12 was the first SQLite WCNG, format 20 introduced the
NODES table, and format 23 is the current WCNG. We have almost no
regression tests for the auto-upgrade code and I believe that the
auto-upgrade code for formats 12 to 19 is broken.

The main failure is that formats 12 to 19 had BASE_NODE/WORKING_NODE
tables and the current SQL queries use the NODES table. There are other
problems, e.g. the 13 to 14 upgrade loses the locked path in the
single-db system.

I'm not interested in fixing these problems. As far as I am concerned
1.7 will support upgrading from 1.6 (and earlier) to the final 1.7.
Anyone who wants to upgrade an intermediate 1.7 format will need to use
an appropriate development client. I'm planning to remove all the
format 12 to format 19 auto-upgrade code, I'll make the code return an
error if such working copies are encountered.

-- 
Philip