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Simplify loading of node class from type string (#2376)

The type string of a `CalcJobNode` should now be just that of the
node class and not the sub class. Since the users are currently still
sub classing the node class and not a process class, we have to make
a special exception when generating the type string for the node.

Conversely, this type string, stored in the `type` column of the node,
should be used to load the `CalcJobNode` class when loading from
the database. The only classes that can legally exist in a database, and
therefore be loaded, are defined in `aiida-core`. Therefore, using the
entry point system to map the type string onto an actual ORM class is no
longer necessary. We rename the `aiida.plugins.loader.load_plugin`
function to the more correct `load_node_class`, which given a type string,
will return the corresponding ORM node sub class.

Note that the whole machinery around generating type and query strings
and loading the nodes based on them is still somewhat convoluted and
contains hacks for two reasons:

 1) Data is not yet moved within the `aiida.orm.node` sub module and as
    a result gets the `data.Data.` type string, which will not match the
    `node.Node.` type when sub classing in queries.

 2) CalcJobProcesses are defined by sub classing JobCalculation
    Until the user directly define a Process sub class that uses the
    `CalcJobNode` as its node class, exceptions will have to be made.

If these two issues are addressed, a lot of the code around type strings
can be simplified and cleaned up.

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