Should refuse SQL transaction statements BEGIN / COMMIT / ROLLBACK from the API

[brodycj]

Related to #863: I think this plugin should refuse to execute BEGIN, COMMIT, and ROLLBACK statements since they may interfere with the existing transaction mechanism, lead to cryptic error messages (see #863), lead to untested & undefined behavior, and maybe even lead to a risk of data loss.

I think this should be in a major version release since it could be a breaking change for some users, including people using TypeORM (see #863 & typeorm/typeorm#4075).

A better long-term solution would probably be a redesigned plugin with a redesigned API (see #548, #862).

[brodycj]

/cc @jacobg