gremlin-javascript: Introduce Connection representation

Currently on gremlin-javascript, DriverRemoteConnection represents a single connection to the server and a RemoteConnection implementation.

Like Python and .NET, we need to make the DriverRemoteConnection use the Client instead of interacting with the websocket directly.

The dependency tree between classes should be like the following

DriverRemoteConnection
|_ Client
  |_ Connection

These changes don't involve a breaking change, its more of an implementation issue. As DriverRemoteConnection is part of the public API the current implementation is a limitation.

This blocks or at least forces awful workarounds for improvements like TINKERPOP-2064.