SAMZA-47 Clean up configuration for key-value store

* stores.my-store.cache.size is now split into two different properties,
  stores.my-store.container.cache.size.bytes (LevelDB block cache size
  in bytes, per container) and stores.my-store.object.cache.size (Samza
  store cache size in number of objects, per task).

* stores.my-store.write.buffer.size is now known as
  stores.my-store.container.write.buffer.size.bytes (semantics changed: was
  measured per-task, now measured per-container)

* stores.my-store.compress is now known as
  stores.my-store.leveldb.compression (semantics changed: was a boolean,
  is now a string indicating the compression codec)

* stores.my-store.batch.size is now known as
  stores.my-store.write.batch.size (unchanged semantics)

* stores.my-store.leveldb.block.size is now known as
  stores.my-store.leveldb.block.size.bytes (unchanged semantics)

Also changed default values for the properties that are now
per-container.

Behaviour of the following configuration properties did not change (but
docs were brought up-to-date with the code):

* stores.my-store.factory
* stores.my-store.changelog
* stores.my-store.key.serde
* stores.my-store.msg.serde
* stores.my-store.object.cache.size

Manually tested changes (using hello-samza). Not yet figured out how to
add automated tests for this.