Allow specifying `SelectorProvider` when constructing an NIO channel #2311

Motivation:

At the moment we use the system-wide default selector provider for this invocation of the Java virtual machine when constructing a new NIO channel, which makes using an alternative SelectorProvider practically useless.
This change allows user specify his/her preferred SelectorProvider.

Modifications:

Add SelectorProvider as a param for current `private static *Channel newSocket` method of NioSocketChannel, NioServerSocketChannel and NioDatagramChannel.
Change default constructors of NioSocketChannel, NioServerSocketChannel and NioDatagramChannel to use DEFAULT_SELECTOR_PROVIDER when calling newSocket(SelectorProvider).
Add new constructors for NioSocketChannel, NioServerSocketChannel and NioDatagramChannel which allow user specify his/her preferred SelectorProvider.

Result:

Now users can specify his/her preferred SelectorProvider when constructing an NIO channel.
Fixing spelling/typo in javadocs for EventExecutor.

Motivation:
Minor spelling/typo correction for EventExecutor's newSucceededFuture and
newFailedFuture javadocs.

Modifications:
Just correcting the spelling/typo.

Result:
Improve the javadocs.
Remove overridden next() method in EventLoop interface.

Motivation:
EventLoop overrides the next() method with the same signature of its
super class EventLoopGroup.

Modifications:
Remove the duplicate method declaration from the EventLoop interface.

Result:
Perhaps makes the API slightly more readable as there is one less
method.