Use BaseConfig instead of raw PluginConfig.

* Pass to the Config class more Gerrit environment variables,
  including AccountCache, PatchListCache, Emails,
  to have more environment-dependent interpretation
  of config key values in the future.
* Use BaseConfig in Config instead of raw PluginConfig
  to allow future extension with more dynamic key values.
* Simplify Checker methods; remove redundant exception specifications.

Change-Id: I50b274aa7220f566288194a8fd219336536f0ac8