Support custom implementor specific graph management

Currently, Tinkerpop uses the [GraphManager](https://github.com/apache/tinkerpop/blob/master/gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/GraphManager.java) to handle graph management and store references to graph objects.

I propose that we change the graphManager in three ways:

1. The graphManager should be an interface, and the existing functionality should be moved into a `BasicGraphManager`. This allows the implementor to design their own graph manager so suit their graph management needs. Furthermore, this will need to allow the implementor to supply a graphManager class in the YAML, so the [Settings](https://github.com/apache/tinkerpop/blob/master/gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/Settings.java) must support a graphmanager, which of course can default to the `BasicGraphManager`.

2. We define additional methods on the interface to improve how the graphManager is used in existing Tinkerpop code. For example, [here](https://github.com/apache/tinkerpop/blob/master/gremlin-server/src/main/java/org/apache/tinkerpop/gremlin/server/util/ServerGremlinExecutor.java#L156) we mutate an object directly outside the scope of a getter or setter on said object.

3. We define additional methods on the interface to allow for custom graph opening implementation:
```
/**

• Implementation that allows for custom graph-opening implementations.
  */
  public Graph openGraph(String graphName, Supplier<Graph> supplier);
  ```