[SPARK-15762][SQL] Cache Metadata & StructType hashCodes; use singleton Metadata.empty

We should cache `Metadata.hashCode` and use a singleton for `Metadata.empty` because calculating metadata hashCodes appears to be a bottleneck for certain workloads.

We should also cache `StructType.hashCode`.

In an optimizer stress-test benchmark run by ericl, these `hashCode` calls accounted for roughly 40% of the total CPU time and this bottleneck was completely eliminated by the caching added by this patch.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #13504 from JoshRosen/metadata-fix.