Simplify the code and only store DataHandler.

- fix an occasional error in the test-case

Query Statistics: new feature which stores the newest 100 SQL queries executed and their performance data.
Useful for tracking down badly performing queries.

The function "regexp_replace" threw the wrong kind of exception if the replacement string was invalid.

Hash indexes can only index one column. Rather throw an error message than silently converting it to a different type of index.

MVStore table engine: new setting "retention_time" to configure the time to retain old data. The default is 45 seconds.