Reroute shards automatically when high disk watermark is exceeded

This adds a Listener interface to the ClusterInfoService, this is used
by the DiskThresholdDecider, which adds a listener to check for nodes
passing the high watermark. If a node is past the high watermark an
empty reroute is issued so shards can be reallocated if desired.

A reroute will only be issued once every
`cluster.routing.allocation.disk.reroute_interval`, which is "60s" by
default.

Refactors InternalClusterInfoService to delegate the nodes stats and
indices stats gathering into separate methods so they have be overriden
by extending classes. Each stat gathering method returns a
CountDownLatch that can be used to wait until processing for that part
is successful before calling the listeners.

Fixes #8146
ok cool