Support for custom createRequest function

[Yogu]

I'd like to instrument createRequest for profiling to find out how long a request has been queued and how long it took to send, wait, receive etc.

Currently, this is very brittle and cumbersome as it's buried in the Connection class which can not easily be extended or changed.

This would also enable more use cases like custom proxies etc, logging, etc. In general, it would be nice to be able to supply some parts of arangojs in some kind of dependency injection (e.g. by passing constructors or functions to the config object), but I think createRequest is the most important one.

To fully support logging and tracing, it would also be good to have some kind of context variable that can be passed to Connection.request and to the wrapper functions like Database.transaction that would be passed to the request function.

[Yogu]

Ah I see, you can pass a custom agent. But still, some kind of context object is still needed so that the request made to the agent can be traced back to a method call in one of the wrapper classes, like Database.transaction.