rename _shard -> _index and also rename classes and variables

closes #4584
Check if node is still present when collecting attribute shard routings

The node we need to lookup for attribute colelction might not be part
of the `DiscoveryNodes` anymore due to node failure or shutdown. This
commit adds a check and removes the shard from the iteration.

Closes #4589
Raise visibilty of `#types()` to public in request classes

`CountRequest` and `ValidateQueryRequest` have package
private accessors for `#types()` which is inconsitent with
other getters.
Fix transient settings assertions in ElasticsearchIntegrationTest

We missed to fail if transient settings were modifed in the test since we
compared against persistent settings.
Add NodeVersionAllocationDecider that prevent allocations that require forward compatibility.

Today during restart scenarios it is possible that we recover from a node that
has already been upgraded to version N+1. The node that we relocate to is
on version N and might not be able to read the index format from the node
we relocate from. This causes `IndexFormatToNewException` during
recovery but only after recovery has finished which can cause large
load spikes during the upgrade period.

Closes #4588
Pass correct number of expected shards to assertion
plugin manager: new `timeout` option

When testing plugin manager with real downloads, it could happen that the test run forever. Fortunately, test suite will be interrupted after 20 minutes, but it could be useful not to fail the whole test suite but only warn in that case.

By default, plugin manager still wait indefinitely but it can be modified using new `--timeout` option:

```sh
bin/plugin --install elasticsearch/kibana --timeout 30s

bin/plugin --install elasticsearch/kibana --timeout 1h
```

Closes #4603.
Closes #4600.
Fix typo in JavaDoc -- s/note/not