PITA having to set WAL provider in two places

With this change if a WAL's meta provider (hbase.wal.meta_provider) is not explicitly set, it now defaults to whatever hbase.wal.provider is set to. Previous, the two settings operated independently, each with its own default. This change is operationally incompatible with previous HBase versions because the default WAL meta provider no longer defaults to AsyncFSWALProvider but to hbase.wal.provider. The thought is that this is more in line with an operator's expectation, that a change in hbase.wal.provider is sufficient to change how WALs are written, especially given hbase.wal.meta_provider is an obscure configuration and that the very idea that meta regions would have their own wal provider would likely come as a surprise.