Fixes regression from 3.x, where sessions where lazy started as required:
Either because an existing session identifier was sent through with the request,
or because new session data needed to be persisted as part of the request execution.
Without this lazy starting, *every* request will get a session,
which makes all those responses uncacheable by HTTP layers.
Note that 4.x also changed the $data vs. $changedData payloads:
In 3.x, they both contained key/value pairs.
In 4.x, $data contains key/value, while $changedData contains key/boolean to declare isChanged.
While this reduces duplication in the class, it also surfaced a bug which was latent in 3.x:
When an existing session is lazily resumed via start(), $data is set back to an empty array.
In 3.x, any changed data before this point was *also* retained in $changedData,
ensuring it gets merged into existing $_SESSION data.
In 4.x, this clears out data - hence the need for a more complex merge logic.
Since isset($this->data) is no longer an accurate indicator of a started session,
we introduce a separate $this->started flag.
Note that I've chosen not to make lazy an opt-in (e.g. via start($request, $lazy=false)).
We already have a distinction between lazy starting via init(), and force starting via start().
At current certain interfaces exist that assume only local assets will be loaded (e.g. `SilverStripe\Forms\HTMLEditor\TinyMCEConfig::getConfig()`), where as someone may wish to load an off site resource via the use of an absolute URL (e.g. for fontawesome css provided via a CDN). Because asset path parsing is filtered through a `SilverStripe\Core\Manifest\ResourceURLGenerator`, one must either know in advance if they want an internal or external resource (loading different generators), or the API must allow for this (i.e. an inclusion function for each type of asset). So we can either double the API on the implementing class, or simply make an exception for an absolute URL as high as possible; inside the filter - for which the `vendor/module : path/to/file.asset` shorthand syntax was specifically designed not to conflict with.
