make setter fields volatile for concurrent decoder modification

[iparadiso]

Summary

• decoder and interpolator in AbstractConfig can be updated at runtime via setDecoder() / setStrInterpolator(). Without volatile, the JVM is not required to flush a write on one thread to main memory, so other threads may observe stale values indefinitely.
• Marking both fields volatile establishes the necessary happens-before relationship: any write is immediately visible to all subsequent reads across threads.
• volatile is the right level of synchronization here — both setters are simple unconditional single-field assignments with no compound read-modify-write, so no additional atomicity guarantee is needed.