Add hint about Galera cluster transactions to avoid deadlocks

[PVince81]

This page mentions Galera: https://doc.owncloud.org/server/9.1/admin_manual/installation/deployment_recommendations.html
Or should be added to "known issues"

See http://severalnines.com/blog/avoiding-deadlocks-galera-set-haproxy-single-node-writes-and-multi-node-reads from owncloud/core#14757 (comment)

@felixboehm @butonic

[felixboehm]

There is also a loadbalancer from mariaDB called maxScale, I recommend to use this as it provides read / write splitting.
https://mariadb.com/kb/en/mariadb-enterprise/mariadb-maxscale/maxscale-readwrite-splitting-with-galera-cluster/

[felixboehm]

A good point in docs to mention this and warn might be here: https://doc.owncloud.com/server/9.0/admin_manual/installation/deployment_recommendations.html#id2

[tomneedham]

https://mariadb.com/kb/en/mariadb-enterprise/consistent-critical-read-filter/ should be configured when using macscale using the exampke configuration:

[CCRFilter]
type=filter
module=ccrfilter
time=5

[tomneedham]

transaction_persistent = 1 in ProxySQL as well