XA - Split the XaResourceManager API from plugin::StorageEngine

Currently, the Resource Manager portion of the X/Open protocol is poorly defined in the plugin::StorageEngine API. All storage engines support a quasi-virtual API for commit(), rollback(), set_savepoint(), etc.

Move this API into a separate plugin::XaResourceManager API and add a plugin::XaStorageEngine shim over this to avoid the multiple plugin class inheritance problem (plugin::getName() is ambiguous)

This piece is essential, as without it all resource managers must implement the entire plugin::StorageEngine API, which is not correct. For instance, the TransactionLog would have to implement the entire StorageEngine API to participate in XA transactions. This is not correct, as the TransactionLog is not a storage engine, but IS an XA Resource Manager...