MVStore: support concurrent transactions (MVCC style, should match the behavior of PostgreSQL, now just read-committed)

found while browsing the code.

Throw an explicit error to make it clear we don't support the TRIGGER combination of SELECT and FOR EACH ROW.

Throw an explicit error to make it clear we don't support the TRIGGER combination of SELECT and FOR EACH ROW.

Throw an explicit error to make it clear we don't support the TRIGGER combination of SELECT and FOR EACH ROW

Apply #418: MySQL compatibility: support for ALTER TABLE tbl_name MODIFY [COLUMN] col_name column_definition, patch from Ville Koskela.

Expose limit/offset/distinct/having/group values as getter methods on Query for external TableEngine implementations

suggestion from markaddleman@gmail.com

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