Extract UnaryOperation from Operation and other changes

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