MastersListener: Fix race conditions around static 'blacklist' map

This fixes two primary issues around this static map:
* The first being that it was a simple 'HashMap' despite being modified and read from multiple threads, possibly concurrently
* The second is to fix a condition where an entry in the blacklist map may be updated, but still removed from 'resetOldsBlackListHosts'
  - This fix is to use the ConcurrentMap functionality to verify the value on the removal, so that if the value has been updated, the removal wont proceed.

Additional there is a slight performance improvement in 'resetOldsBlackListHosts' by both not needing to copy the key set, as well as iterating over the entry set to avoid the additional .get() call to get the stored time value.

src/main/java/org/mariadb/jdbc/internal/failover/AbstractMastersListener.java

@@ -61,10 +61,9 @@ WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWIS
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.sql.SQLException;
-import java.util.HashMap;
-import java.util.HashSet;
 import java.util.Map;
-import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledFuture;
@@ -79,7 +78,7 @@ public abstract class AbstractMastersListener implements Listener {
     /**
      * List the recent failedConnection.
      */
-    protected static Map<HostAddress, Long> blacklist = new HashMap<>();
+    protected static ConcurrentMap<HostAddress, Long> blacklist = new ConcurrentHashMap<>();
     /* =========================== Failover variables ========================================= */
     public final UrlParser urlParser;
     protected AtomicInteger currentConnectionAttempts = new AtomicInteger();
@@ -151,12 +150,12 @@ public void addToBlacklist(HostAddress hostAddress) {
      */
     public void resetOldsBlackListHosts() {
         long currentTime = System.currentTimeMillis();
-        Set<HostAddress> currentBlackListkeys = new HashSet<HostAddress>(blacklist.keySet());
-        for (HostAddress blackListHost : currentBlackListkeys) {
-            if (blacklist.get(blackListHost) < currentTime - urlParser.getOptions().loadBalanceBlacklistTimeout * 1000) {
-//                if (log.isTraceEnabled()) log.trace("host " + blackListHost+" remove of blacklist");
-                blacklist.remove(blackListHost);
-            }
+        for (Map.Entry<HostAddress, Long> blEntry : blacklist.entrySet()) {
+            long entryTime = blEntry.getValue();
+            if (entryTime < currentTime - urlParser.getOptions().loadBalanceBlacklistTimeout * 1000) {
+//              if (log.isTraceEnabled()) log.trace("host " + blackListHost+" remove of blacklist");
+              blacklist.remove(blEntry.getKey(), entryTime);
+          }
         }
     }