[SPARK-2485][SQL] Lock usage of hive client.

Since we just had a problem with this in Spark core, I want to point out that locking other people's objects can lead to deadlock if they use synchronization on `this` internally. Indeed, this Hive object seems like a relatively prime candidate for this type of trouble since it's gotten through a static constructor (so is cache-able) and it's Hive (so is probably not inherently good at thread safety).

Just pointing this out, perhaps it's not a risk in this case.