Retains h2 database connection only during dev mode

https://github.com/playframework/Play20/pull/688 caused our integration tests to fail given that H2 databases were being kept open across the invocation of integration tests. An improved solution has been determined on the basis that:

1. Unit tests conventionally expect that a db is cleared for every test invocation
2. Dev mode requires that a db is kept open

I have rolled back the h2 db closing changes of https://github.com/playframework/Play20/pull/688 but have retained the Evolutions fix. Also, for h2 in dev mode, we now automatically append the DB_CLOSE_DELAY=-1 if it has not already been specified by configuration.