Slightly reduce the memory cost of View metadata, helps with people who have lots and lots of views, and there is no real performance gain in caching the plan SQL string

The collation can now be set in the database URL, even if there are data tables, if the collection is the same as the current collation.

Added support for ON DUPLICATE KEY UPDATE like MySQL with the values() function to update with the value that was to be inserted.
Patch from Jean-Francois Noel.

simplify background writer thread implementation, tying the sync object lifetime more closely to the thread object lifetime