Implement AutoCloseable in Channel

[boryspiela]

It would be nice to implement AutoCloseable interface in Channel class and override its' close method.
AutoCloseable implementation was previously done in #131 in Connection class but Channel was left with a comment "I wanted to add it to Channel-class also but it had a checked exception TimedOutException that broke the contract of Closeable so I avoided it." (#133) but there is no specification for close method to not throw java.util.concurrent.TimeoutException. close method should not throw java.lang.InterruptedException.

[michaelklishin]

The docs seem to concur. Now that we are working on 5.0 targeting JDK 8, we can reconsider this.

[zaharidichev]

Can i have a take on that one ?

[michaelklishin]

@zaharidichev sure, why not. Take a look at CONTRIBUTING.md. I believe this change is small enough to not require a signed CA but if you could file on, it'd be particularly appreciated. Thank you!