#1245 - Refactor internals - Add to TransactionManager externalRemoveTransaction() and externalBeginTransaction()

These provide a better/tidier way for external transaction managers (like ebean-spring-txn) to begin and end externally
managed transactions.

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

@@ -0,0 +1,37 @@
+package io.ebeaninternal.api;
+
+import io.ebean.TxScope;
+
+import javax.sql.DataSource;
+
+/**
+ * Service provider interface for the transaction manager.
+ */
+public interface SpiTransactionManager {
+
+  /**
+   * Return the main DataSource.
+   */
+  DataSource getDataSource();
+
+  /**
+   * Return the read only DataSource (if defined).
+   */
+  DataSource getReadOnlyDataSource();
+
+  /**
+   * Return the currently active transaction (can be null).
+   */
+  SpiTransaction get();
+
+  /**
+   * Push an externally managed transaction into scope (e.g. Spring managed transaction).
+   */
+  ScopedTransaction externalBeginTransaction(SpiTransaction transaction, TxScope txScope);
+
+  /**
+   * Called when an externally managed transaction has completed.
+   */
+  void externalRemoveTransaction();
+
+}