Corrected calculation of the next MyMoneyObject ids in SQL backend

The next MyMoneyObjects' ids were stored in the table kmmFileInfo. But
this data got corrupted in the past several times.

Now the highest id is calculated - so corruptions should not be possible
anymore.

The calculation process is lazy and is only executed when needed as it
is quite expensive (uses regex on some RDBMSs). Unfortunatly the SQL
backend triggers the calculation all the time at the moment - way more
often than needed. Some writeFileInfo() and readFileInfo() calls must
be removed to fix that (future work).

Any calls to writeFileInfo() and readFileInfo() are removed - these
methods cannot be called before the upgrade process is finished!

Also the higest id numbers are not set during the upgrade process.

BUG: 339103
FIXED-IN: 4.8