Immutable statements

Honor omitempty on InsertInto(). Closes #297

WIP: Better support for raw queries and subqueries.

Make prepared statement cache optional (and disabled by default)

Interpolate subqueries with ?

Modify Makefile to run tests concurrently

Adds per-session SetConnMaxLifetime, SetMaxIdleConns and SetMaxOpenConns and prevents transactions from being prepared concurrently on the same session.

Add default AND join within Or. Closes #280

Fix handling of slice parameters. Closes #279

Add tests that count prepared statements during testing

Add test for Where() before Set() on update

Don't use statements that are marked as dead