If we have multiple cores, leave one free from crypto tasks.

bramble-core/src/main/java/org/briarproject/bramble/crypto/CryptoModule.java

@@ -39,7 +39,7 @@ public class CryptoModule {
 	 * The maximum number of executor threads.
 	 */
 	private static final int MAX_EXECUTOR_THREADS =
-			Runtime.getRuntime().availableProcessors();
+			Math.max(1, Runtime.getRuntime().availableProcessors() - 1);
 
 	private final ExecutorService cryptoExecutor;
 

bramble-core/src/main/java/org/briarproject/bramble/sync/SyncModule.java

@@ -36,7 +36,7 @@ public class SyncModule {
 	 * executor concurrently.
 	 */
 	private static final int MAX_CONCURRENT_VALIDATION_TASKS =
-			Runtime.getRuntime().availableProcessors();
+			Math.max(1, Runtime.getRuntime().availableProcessors() - 1);
 
 	@Provides
 	GroupFactory provideGroupFactory(CryptoComponent crypto) {