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

Add And() to Result

Add NumActiveStatements() and test case