Support for Channel specification on ServiceClient

[rebello95]

Adds support to allow consumers to specify a Channel when creating ServiceClient instances.

This is something already supported by other gRPC language flavors (like Python), and alleviates the need to create a new channel every time a call is made.

Usage:

let sharedChannel = Channel(address: "https://api.myservice.com", secure: true)
HelloWorldClient(channel: sharedChannel).getHelloWorld(...)
OtherClient(channel: sharedChannel).getOther(...)

Resolves #200

[rebello95]

@MrMage if you'd like to take a look 😀

Thought about adding additional tests for this as well, but it looks like reusing channels is already covered here. Let me know if you think there's something I should add!

[MrMage]

LGTM

Thanks, Michael! This is indeed sort-of covered by gRPCTests, but those tests are very generic and tests many things at once. So I would indeed appreciate a small dedicated test (in EchoTests or a new file ServiceClientTests, I'd say) that specifically tests using two ServiceClients with the same channel. Something like this:

let channel = Channel(...)
do {
    let client = EchoClient(channel: channel)
    XCTAssertEqual("foo", client.get(...).text)
}
// First client has left scope by now.
do {
    let client = EchoClient(channel: channel)
    XCTAssertEqual("foo", client.get(...).text)
}

Hope that makes sense.

[MrMage]

I think we can remove this gRPC.initialize() call, given that this is now done by Channel.init. Also in the other constructors above and below.

Also, I'd suggest moving this constructor below or above all the other ones?

[timburks]

LGTM.

Thanks @rebello95 for adding this, and @MrMage for reviewing! I agree that future tests would be important for this, but will merge this now.

[rebello95]

Thank you for the feedback @MrMage and @timburks! As this is already merged, I went ahead and wrote some tests in #203 that should cover it