Do not consider PortUnreachableException to require channel closure

Motivation:

When a datagram packet is sent to a destination where nobody actually listens to,
the server O/S will respond with an ICMP Port Unreachable packet.
The ICMP Port Unreachable packet is translated into PortUnreachableException by JDK.
PortUnreachableException is not a harmful exception that prevents a user from sending a datagram.
Therefore, we should not close a datagram channel when PortUnreachableException is caught.

Modifications:

- Do not close a channel when the caught exception is PortUnreachableException.

Result:

A datagram channel is not closed unexpectedly anymore.
Add EDNS support to DnsQueryEncoder

Motivation:

DnsQueryEncoder does not encode the 'additional resources' section at all, which contains the pseudo-RR as defined in RFC 2671.

Modifications:

- Modify DnsQueryEncoder to encode the additional resources
- Fix a bug in DnsQueryEncoder where an empty name is encoded incorrectly

Result:

A user can send an EDNS query.
Name resolver API and DNS-based name resolver

Motivation:

So far, we relied on the domain name resolution mechanism provided by
JDK.  It served its purpose very well, but had the following
shortcomings:

- Domain name resolution is performed in a blocking manner.
  This becomes a problem when a user has to connect to thousands of
  different hosts. e.g. web crawlers
- It is impossible to employ an alternative cache/retry policy.
  e.g. lower/upper bound in TTL, round-robin
- It is impossible to employ an alternative name resolution mechanism.
  e.g. Zookeeper-based name resolver

Modification:

- Add the resolver API in the new module: netty-resolver
- Implement the DNS-based resolver: netty-resolver-dns
  .. which uses netty-codec-dns
- Make ChannelFactory reusable because it's now used by
  io.netty.bootstrap, io.netty.resolver.dns, and potentially by other
  modules in the future
  - Move ChannelFactory from io.netty.bootstrap to io.netty.channel
  - Deprecate the old ChannelFactory
  - Add ReflectiveChannelFactory

Result:

It is trivial to resolve a large number of domain names asynchronously.
Fix test failures in ProxyHandlerTest

Motivation:

The default name resolver attempts to resolve the bad host name (destination.com) and actually succeeds, making the ProxyHandlerTest fail.

Modification:

Use NoopNameResolverGroup instead.

Result:

ProxyHandlerTest passes again.