Today `node.mode` and `node.local` serve almost the same purpose, they
are a shortcut for `discovery.type` and `transport.type`. If `node.local: true`
or `node.mode: local` is set elasticsearch will start in _local_ mode which means
only nodes within the same JVM are discovered and a non-network based transport
is used. The _local_ mode it only really used in tests or if nodes are embedded.
For both, embedding and tests explicit configuration via `discovery.type` and `transport.type`
should be preferred.
This change removes all the usage of these settings and by-default doesn't
configure a default transport implemenation since netty is now a module. Yet, to make
the user expericence flawless, plugins or modules can set a `http.type.default` and
`transport.type.default`. Plugins set this via `PluginService#additionalSettings()`
which enforces _set-once_ which prevents node startup if set multiple times. This means
that our distributions will just startup with netty transport since it's packaged as a
module unless `transport.type` or `http.transport.type` is explicitly set.
This change also found a bunch of bugs since several NamedWriteables were not registered if a
transport client is used. Now that we don't rely on the `node.mode` leniency which is inherited
instead of using explicit settings, `TransportClient` uses `AssertingLocalTransport` which detects these problems since it serializes all messages.
Closes#16234
5616251f22