Netty: Make sure channel closing never happens on i/o thread

Similar to NettyTransport.doStop() all actions which disconnect
from a node (and thus call awaitUnterruptibly) should not be executed
on the I/O thread.

This patch ensures that all disconnects happen in the generic threadpool, trying to avoid unnecessary `disconnectFromNode` calls.

Also added a missing return statement in case the component was not yet
started when catching an exception on the netty layer.

Closes #7726
Tools: Add script to grab ES version for BWC tests.

closes #7653
Internal: ClusterHealthAPI does not respect waitForEvents when local flag is set

It uses a cluster state update task and it gets rejected if not run on a master node. We should enable running on non-masters if the local flag is set.

Also, report any unexpected error that may happen during this cluster state update task

Closes #7731
Discovery: not all master election related cluster state update task use Priority.IMMEDIATE

Most notably the elected_as_master task should run as soon as possible. This is an issue as node join request do use `Priority.IMMEDIATE` and can be unjustly rejected.

Closes #7718
Aggs - support for arrays of numeric values in include/exclude clauses

Closes #7714