Fixed PHP-1278: runcommand memory leak
Merged pull request #770
PHP-1258: Run the testsuite against alternative storage engines
Make the setting of storage engines actually work.
Also allow storage engines to be set by an environment variable so no mucking of config file is necessary.

Fits in nicely with the MONGO_SERVER_* env vars too.
Don't forget the master/slave configuration.
Also set the storage engine for the config server(s).