Stop reading the channel when bandwidth limit if reached

@fredericBregier @normanmaurer 
previous discussion on this problem: https://github.com/netty/netty/pull/2494#issuecomment-49735979

when isSuspended return false, ctx.read will make the channel readable, and otherwise the channel is not readable, following this logic, there may be something wrong: 
1. ctx.read will call AbstractNioChannel#doBeginRead, and doBeginRead will add readInterestOps to the channel, and thus the channel is readable, but
2. if you don't call ctx.read when isSuspended return true, the readInterestOps is NOT removed from the channel, the channel is still readable;

we should call ctx.channel().config().setAutoRead(false) when isSuspended return true.