test(Files): Add integration tests for Files.copy / read*

[guicamest]

Issue #236

Description of changes:

As discussed in the issue, a non-crt asyncclient will be used for reading files through the S3ReadAheadByteChannel. This is the class used when using the nio api via Files.copy or Files.read*.

Some private methods have been renamed to better reflect the purpose of them.

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[guicamest]

While doing the change I came across universalClient method(s), I haven't found a purpose for them and thought of cleaning them out. @markjschreiber would that be ok?

[markjschreiber]

Any reason why you wouldn't just overload client() with client(boolean crtClient)? Either way, it is probably worth documenting why a CRT client is not used in this case.

[guicamest]

The client() method saves a reference of that client in the current class. Unlike client(), readClient() creates a new client every time a read needs to be done. This client is later closed when the readChannel is close(). I think it'd be clearer to have them in separate methods for now, one is a general purpose and the other one is only for reads.

[markjschreiber]

Just one minor question. Otherwise looking good.