Add --no-autocommit when dumping MySQL tables

Very large databases can take a long time to load, in part because indexes are regenerated after each insert, which is useless when simply loading a database dump, as happens when provision clones or migrates a site.

By default mysqldump is already using the options unique_checks=0, foreign_key_checks=0, and disable keys (although the latter has no effect on InnoDB tables, the D7 default). But we should add --no-autocommit, which wraps each table as follows:

set autocommit=0;
INSERT ...
INSERT ...
INSERT ...
commit;

Assigning version 7.x-3.x, but backport to 6.x-2.x should be trivial.