Fix upgrade.php --with-new-sql failures on source with database migrations

Prior to this commit, when a developer pulled the source code and there was a database migration since the last stable release, php upgrade --with-new-sql potentially failed because that migration was already published in build-db_mysql.sql, which was already run during installation.

This commit reverts build-db_mysql.sql to the latest version release, and creates a new script, build-db_mysql-upcoming-release.sql, which tests use. When the developer installs from source, s/he must also then run php upgrade --with-new-sql afterward, which will apply any pending migrations which have not gone out in a new user release.

From now on, when a new user release gets packaged up, every change in the upcoming-release sql script will get rolled into build-db_mysql.sql.