It's read for every command execution, and during slave replication
for every applied event.

It's also planned to be used during write set applying, so it means
mostly every server thread is going to compete for the mutex covering
this variable, especially considering how rarely it changes.
Converting wsrep_ready to atomic relaxes the things.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>