Encode chunks off the main thread.
Limit the number of concurrent savings to 1.
Ported some StorageManagerInternal tests to the new API.
Make StorageManagerTest properly test the storage of zipped chunks.

It no longer extends from TerasologyTestingEnvironment as the creation
of the zip file system doesn't work ontop of a fake file system.
Simplified some StorageManagerTests by doing more in the test setup method.
Port the remaining StorageManagerTests to the new API.