refactor jca subordinate transaction support

The JTA and JTAX both have similar support for JCA tx inflow, using TxImporter, XATerminator and TransactionImple classes. However, these have little in common beyond the name, making it difficult for user code to switch between running the JTA or JTAX versions. Refactor to introduce common interfaces & such, so that client code can exploit polymorphism rather than using way too many if/else constructs.