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.
[#2339] Reduce memory usage in ProtobufVarint32LengthFieldPrepender

Motivation:

Reduce memory usage in ProtobufVarint32LengthFieldPrepender.

Modifications:

Explicit set the buffer size that is needed for the header (between 1 and 5 bytes).

Result:

Less memory usage in ProtobufVarint32LengthFieldPrepender.