With some type of load, such as one statement "light" transactions, the user xa may perform much poorer on the optimistic parallel slave than an equivalent normal - BEGIN..COMMIT - load.
This ticket aims at fixing the slowness while the XA high-availability is to be preserved.
The whole work is arranged in two parts:
1. refactoring of the user xa binary logging to facilitate slave side parallel execution
2. the slave side change of the parallel scheduler to apply Round-Robin distribution
for the user XA transactions.