Specifying `?master_timeout=-1` on an API which performs a cluster state
update means that the cluster state update task will never time out
while waiting in the pending tasks queue. However this parameter is also
re-used in a few places where a timeout of `-1` means something else,
typically to timeout immediately. This commit fixes those places so that
`?master_timeout=-1` consistently means to wait forever.
Makes several changes to consolidate snapshot and backup-related docs.
Highlights:
* Adds info about supported ESS snapshot repository types
* Adds docs for Kibana's Snapshot and Restore feature
* Combines tutorial pages related to taking and managing snapshots
* Consolidates explanations of the snapshot process
* Incorporates SLM into the snapshot tutorial
* Removes duplicate "back up a cluster" pages
