Fix leaked db connections in SpringTransactionManager

[amseager]

Fixes #134

Thanks to @GrannyOlli and @kzkvv for pointing it out.

To sum up: the method inTransactionReturnsThrows of SpringTransactionManager is marked as @Transactional but this annotation takes effect only if this method is called outside its class (e.g. from here).

Currently, there are 4 default methods of TransactionManager which both call it too, and, in fact, from the same class (SpringTransactionManager inherits this interface), so @Transactional doesn't work in this case because of Spring AOP restrictions. In some cases, this can cause errors (e.g. a connection can be left open after an exception occurring inside the transaction).

In this PR, I overrode those methods with the same implementations but also added @Transactional for them. So now, a new transaction will be created while calling them too. On the other hand, inTransactionReturnsThrows can be safely called inside those methods because, in this case, its @Transactional is ignored and there won't be extra transactions spawned (as it could be if we call interface default methods with the super keyword).

Another solution would be to move inTransactionReturnsThrows to a different class, but since it's a Spring-only related issue, I think it's better to leave it as is.

[badgerwithagun]

Great! I will take the Spring users' word for this. Looks good.