WALFactory has misleading notion of "default"

In WALFactory, there is an enum Providers which has a list of supported WALProvider implementations. In addition to list this, there is also a defaultProvider (which the Configuration defaults to), that is meant to be our "advertised" default WALProvider.

However, the implementation of getProviderClass in WALFactory doesn't actually adhere to the value of this enum, instead always returning AsyncFSWal if it can be loaded.

Having the default value in the enum but then overriding it in the implementation of getProviderClass is silly and misleading.