Fix missing version properties of transport-epoll in all-in-one JAR

Related: #2952

Motivation:

META-INF/io.netty.versions.properties in netty-all-*.jar does not
contain the version information about the netty-transport-epoll module.

Modifications:

Fix a bug in the regular expression in pom.xml, so that the artifacts
with a classifier is also included in the version properties file.

Result:

The version information of all modules are included in the version
properties file, and Version.identify() does not miss
netty-transport-epoll.
Netty Headers Class Restructure and Algorithm Updates

Motivation:
Headers within netty do not cleanly share a common class hierarchy.  As a result some header types support some operations
and don't support others.  The consolidation of the class hierarchy will allow for maintenance and scalability for new codec.
The existing hierarchy also has a few short comings such as it is not clear when data conversions are happening.  This
could result unintentionally getting back a collection or iterator where a conversion on each entry must happen.

The current headers algorithm also prepends all elements which means to find the first element or return a collection
in insertion order often requires a complete traversal followed by a collections.reverse call.

Modifications:
-Provide a generic base class which provides all the implementation for headers in netty
-Provide an extension to this class which allows for name type conversions to happen (to accommodate legacy CharSequence to String conversions)
-Update the headers interface to clarify when conversions will happen.
-Update the headers data structure so that appends are done to avoid unnecessary iteration or collection reversal.

Result:
-More unified class hierarchy for headers in netty
-Improved headers data structure and algorithms
-headers API more clearly identify when conversions are required.
CorsHandler should release HttpRequest after processing preflight/error.

Motivation:
Currently, when the CorsHandler processes a preflight request, or
respondes with an 403 Forbidden using the short-curcuit option, the
HttpRequest is not released which leads to a buffer leak.

Modifications:
Releasing the HttpRequest when done processing a preflight request or
responding with an 403.

Result:
Using the CorsHandler will not cause buffer leaks.
Handle an empty ByteBuf specially in HttpObjectEncoder

Related: #2983

Motivation:

It is a well known idiom to write an empty buffer and add a listener to
its future to close a channel when the last byte has been written out:

  ChannelFuture f = channel.writeAndFlush(Unpooled.EMPTY_BUFFER);
  f.addListener(ChannelFutureListener.CLOSE);

When HttpObjectEncoder is in the pipeline, this still works, but it
silently raises an IllegalStateException, because HttpObjectEncoder does
not allow writing a ByteBuf when it is expecting an HttpMessage.

Modifications:

- Handle an empty ByteBuf specially in HttpObjectEncoder, so that
  writing an empty buffer does not fail even if the pipeline contains an
  HttpObjectEncoder
- Add a test

Result:

An exception is not triggered anymore by HttpObjectEncoder, when a user
attempts to write an empty buffer.
Overall clean-up on Headers and its subtypes

Motivation:

- There are still various inspector warnings to fix.
- ValueConverter.convert() methods need to end with the type name like
  other methods in Headers, such as setInt() and addInt(), for more
  consistency

Modifications:

- Fix all inspector warnings
- Rename ValueConverter.convert() to convert<type>()

Result:

- Cleaner code
- Consistency
Headers Cleanup

Motiviation:
Some of the contains method signatures did not include the correct type of parameters for the method names.  Also these same methods were not using the correct conversion methods but instead the generic object conversion methods.

Modifications
-Correct the parameter types of the contains methods to match the method name
-Correct the implementation of these methods to use the correct conversion methods

Result:
Headers contains signatures are more correct and correct conversion methods are used.