PriceProvider: Add volatile to cachedResult field

The get() and put() calls can happen on different threads. The volatile
keyword makes sure that all updates to cachedResult are visible to all
threads.

Relates to bisq-network#33.

src/main/java/bisq/price/PriceProvider.java

@@ -39,7 +39,7 @@ public abstract class PriceProvider<T> implements SmartLifecycle, Supplier<T> {
 
     protected final Duration refreshInterval;
 
-    private T cachedResult;
+    private volatile T cachedResult;
 
     public PriceProvider(Duration refreshInterval) {
         this.refreshInterval = refreshInterval;