Add stmt.prepare, stmt.execute, stmt.cancel

* Prepare and execute statements, cancel statments in SQL_NEED_DATA
state (possibly could be used to cancel running operations, needs more
research).
* Export OdbcError
* Fix bug when last callback is called, new callbacks are registered,
and last callback is called again (stack not reset properly)
* Remove Eos::Unwait (no longer used)
* Some basic tests for Connection and Statement