Avoid multiple commands in transaction code

Avoid multiple commands in transaction code

In pipeline mode, command strings containing multiple SQL commands are
disallowed so we remove all such usages from transaction code.

Accordingly, all generator methods in transaction classes now do not
return anything (the result was not used previously anyways).

In tests, the 'commands' list defined in patch_exec() is now filled by
appending instead of inserting so that we keep the natural order of
commands in assertions.