Support for detaching prepared XA transactions

Add support for detaching XA transactions. This is useful for handling
the case where the DBMS client has a transaction in prepared state and
disconnects. Before disconnect, the DBMS calls the newly introduced
client_state::xa_detach(), to cleanup the local transaction and
convert it to a high priority transaction. The DBMS may later attempt
to terminate the transaction through client_state::commit_by_xid() or
client_state::rollback_by_xid().

Also in this patch:

- Fix client_state::close() so that it does not rollback transactions
  in prepared state
- Changed class wsrep::xid representation to hold enough information
  so that DBMS can convert to its native representation
- Fix potential infinite loop in
  server_state::find_streaming_applier(wsrep:xid&)
- Append SR keys on prepare fragment and make it pa_unsafe
- Handle one phase commit (simply fall back to two phase)
- Do not rollback prepared streaming clients in
  server_state::close_orphaned_transactions()

src/client_state.cpp

@@ -50,7 +50,9 @@ void wsrep::client_state::close()
     debug_log_state("close: enter");
     state(lock, s_quitting);
     lock.unlock();
-    if (transaction_.active())
+    if (transaction_.active() &&
+        (mode_ != m_local ||
+         transaction_.state() != wsrep::transaction::s_prepared))
     {
         client_service_.bf_rollback();
         transaction_.after_statement();