Comment 3 for bug 1361859

As description of 'wsrep_causal_reads' said: "(DEPRECATED) setting this variable is equivalent to setting wsrep_sync_wait READ flag". So we not only have to update session 'wsrep_sync_wait' if we update session 'wsrep_causal_reads', we have to do it to global variables too. And vice versa.

if we don't update global values, it will bring inconsistency and introduce confusion to users. For example, if we don't update global values, there would be scenario:
- global wsrep_causal_reads = 1
- global wsrep_sync_wait = 6
from wsrep_causal_reads, we DO expect causality check on READ stmt. However from wsrep_sync_wait, we DO NOT expect causality check on READ stmt.