Add initial implementation of WITH TIES clause

Update changelog and fix building of documentation

Add initial DISTINCT ON (expression, ...) implementation

Handle distinct results properly

This reverts commit 6933c751.

Assorted minor changes

Move HELP and SHOW tests into own files

Fix update count for REPLACE and move some SQL tests into separate files

Add Function "LEN" #1196

Do not load the whole LOBs into memory for comparison operation