Fix wsrep XID storage byte order

The sequence number part of wsrep XID is currently stored in host byte order into XID data buffer. This will become a problem for example in a case when a physical backup is prepared on a machine with big endian architecture and is recovered on a machine with little endian architecture. The recovered sequence number will have wrong value.

A solution for this problem is to store wsrep XID sequence number part in little endian byte order into XID data buffer by using ```int8store()``` and to read the seqno part with ```sint8korr()```.

In order to keep backwards compatibility, the seqno should be read in host byte order from TRX_SYS page when upgrading.