[SQL] Make functionRegistry in HiveContext transient.

Seems we missed `transient` for the `functionRegistry` in `HiveContext`.

cc: marmbrus

Author: Yin Huai <huaiyin.thu@gmail.com>

Closes #2074 from yhuai/makeFunctionRegistryTransient and squashes the following commits:

6534e7d [Yin Huai] Make functionRegistry transient.