Prevent channel re-registration from firing channelActive

Motivation:

AbstractUnsafe considers two possibilities during channel registration. First,
the channel may be an outgoing connection, in which case it will be registered
before becoming active. Second, the channel may be an incoming connection in,
which case the channel will already be active when it is registered. To handle
the second case, AbstractUnsafe checks if the channel is active after
registration and calls ChannelPipeline.fireChannelActive() if so.  However, if
an active channel is deregistered and then re-registered this logic causes a
second fireChannelActive() to be invoked. This is unexpected; it is reasonable
for handlers to assume that this method will only be invoked once per channel.

Modifications:

This change introduces a flag into AbstractUnsafe to recognize if this is the
first or a subsequent registration. ChannelPipeline.fireChannelActive() is only
possible for the first registration.

Result:

ChannelPipeline.fireChannelActive() is only called once.
@buchgr  OK, I'll reorder them.  My thinking was that `neverRegistered` is only ever accessed on the event loop so that would give us enough threadsafety (the same cannot be said of `registered`).  But there is certainly no harm in reordering them.