gRPC client factory integration

[czimpi]

Currently the DgraphClient class only exposes a constructor accepting gRPC channels, which will then be used to create the gRPC clients. These channels have to be manually mantained (lifetime, etc.) which may lead to misuses (e.g. gRPC channels should not be created for each new request but rather reused, see docs).

Using the gRPC client factory integration is an alternative approach to outsource the maintenance of the channel respectively the client. To use this approach, the DgraphClient class must accept the gRPC client directly rather than channels.

The current implementation does add the client from the constructor to the list of clients which are subject to the (currently not active) dispose logic. This should not be a problem, as the logic is currently inactive, but in the future this might be an issue, as the client's lifetime management should be managed by the client factory. I personally would also remove the inactive dispose logic and leave the channel lifetime management to the caller of the DgraphClient.

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[MichelDiz]

@czimpi can you update this PR and also sign the CLA?

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