Retore immutability/thread-safety to ArchiveStreamFactory

COMPRESS-180 added support for encoding.
Unfortunately this was done in a way that broke immutability.
Also the factory is no longer thread-safe as the encoding field is not synch/volatile.

Consider whether to restore immutability, e.g. by adding a constuctor which takes the encoding setting. The setEntryEncoding method could be deprecated intitially and eventually dropped.

One way to support immutability now would be to add a second final encoding field which is set by a new ctor. See patch to follow.