Refactor DefaultTransactionThreadLocal, convert ThreadLocal to field (from map) (#1716)

* Refactor DefaultTransactionThreadLocal, convert ThreadLocal to field (from map)

* Add test for multiple database thread locals

src/main/java/io/ebeaninternal/api/SpiTransactionManager.java

@@ -1,6 +1,7 @@
 package io.ebeaninternal.api;
 
 import io.ebean.TxScope;
+import io.ebeaninternal.server.transaction.TransactionScopeManager;
 
 import javax.sql.DataSource;
 
@@ -9,6 +10,11 @@ import javax.sql.DataSource;
  */
 public interface SpiTransactionManager {
 
+  /**
+   * Return the scope manager for this server.
+   */
+  TransactionScopeManager scope();
+
   /**
    * Return the main DataSource.
    */