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
Take percentage watermarks into account for reroute listener

Fixes an issue where only absolute bytes were taken into account when
kicking off an automatic reroute due to disk usage. Also randomized the
tests to use either an absolute value or a percentage so this is tested.

Also adds logging for each node over the high and low watermark every
time a new cluster info usage is gathered (defaults to every 30
seconds).

Related to #8368
Fixes #8367