Historically we have loaded SSL objects (such as SSLContext,
SSLIOSessionStrategy) by passing in the SSL settings, constructing a
new SSL configuration from those settings and then looking for a
cached object that matches those settings.

The primary issue with this approach is that it requires a fully
configured Settings object to be available any time the SSL context
needs to be loaded. If the Settings include SecureSettings (such as
passwords for keys or keystores) then this is not true, and the cached
SSL object cannot be loaded at runtime.

This commit introduces an alternative approach of naming every cached
ssl configuration, so that it is possible to load the SSL context for
a named configuration (such as "xpack.http.ssl"). This means that the
calling code does not need to have ongoing access to the secure
settings that were used to load the configuration.

- Prevent dynamic changes to SSL settings that also use secure
  settings
- Dyanmically rebuild SSL context for exporters that don't use
  secure setting

There's a few places where the usage is difficult to remove, so I'm
leaving them for now and we can tackle them another time.