Do not abort prepared transaction when statement is interrupted

In `transaction::abort_or_interupt()` handle the case were a statement
is interrupted while transaction is in prepared state. In which case,
the transaction must remain in prepared state, until it is
committed (or rolled back).
Also in this patch: remove check for reduntant `s_must_abort` state.

Author: sciascid

src/transaction.cpp

@@ -1400,10 +1400,19 @@ bool wsrep::transaction::abort_or_interrupt(
         }
         return true;
     }
-    else if (client_service_.interrupted(lock))
+
+    if (client_service_.interrupted(lock))
     {
+        assert(state() != s_must_abort &&
+               state() != s_aborting &&
+               state() != s_aborted);
+
+        // Client was interrupted. Set the appropriate error and abort.
+        // For transactions in prepared state, it is OK to interrupt the
+        // statement, but transaction must remain in prepared state until
+        // commit or rollback.
         client_state_.override_error(wsrep::e_interrupted_error);
-        if (state() != s_must_abort)
+        if (state() != s_prepared)
         {
             state(lock, s_must_abort);
         }