Update upgrading.md

Adding in redis FLUSHALL
typo: "even" --> "event"
Added first draft of Scaling Strategies under an Advanced Topics heading
Merge pull request #128 from solarkennedy/clustering_docs

Added first draft of Scaling Strategies under an Advanced Topics heading
Merge pull request #126 from chrisarnesen/patch-1

typo: "even" --> "event"
Merge pull request #124 from dillera/patch-2

Update upgrading.md