Fix missing true return lost during e5a62c46f

This never resulted in a serious problem because it was papered over by a
bug in Class::C3::Componentised. Addressing it at the source requires a bit
extra coding, hence no new CC3C release yet.
Minimal optimization of the new+prefetch codepath (no func. changes)
Clarify that sql_maker is just a getter at this point
Properly detect and test mysql v3 default JOIN behavior

While investigating how to rewrite the rather useless 39712b481, it became
clear that not only can the test be performed offline, but that there is
already a pseudo-test due to sqlite's version of 3.x.y being mistaken for
MySQL 3

Properly fix the test to check behavior on both versions 3 and 4, and in the
process fix a potential bug of a stale sqlmaker when a reconnect cycle takes
place against a *different* physical rdbms (which btw makes me realize another
problem with trying to do mixed-environment replication... sigh)