DISK_SPACE_USED() for MVStore and other minor changes

split up the rather large convertTo method

and unify the convert-from-string code with the corresponding target
type, so the logic is easy to see on a per-target-type basis.

Throw exception if unknown mode is specified in database URL

Parse more date-time literals for compatibility with other databases

Minor fixes for INTERVAL data type

TestMultiThreadedKernel is back

Improve documentation of intervals

Add more methods to Interval and remove h2.unlimitedTimeRange

Fix MERGE USING with parameters