Refactor integration tests of compression codecs

Motivation:

Duplicated code of integration tests for different compression codecs.

Modifications:

- Added abstract class IntegrationTest which contains common tests for any compression codec.
- Removed common tests from Bzip2IntegrationTest and LzfIntegrationTest.
- Implemented abstract methods of IntegrationTest in Bzip2IntegrationTest, LzfIntegrationTest and SnappyIntegrationTest.

Result:

Removed duplicated code of integration tests for compression codecs and simplified an addition of integration tests for new compression codecs.
Refactor Bzip2 tests

Motivation:

Complicated code of Bzip2 tests with some unnecessary actions.

Modifications:

- Reduce size of BYTES_LARGE array of random test data for Bzip2  tests.
- Removed unnecessary creations of EmbeddedChannel instances in Bzip2 tests.
- Simplified tests in Bzip2DecoderTest which expect exception.
- Removed unnecessary testStreamInitialization() from Bzip2EncoderTest.

Result:

Reduced time to test the 'codec' package by 7 percent, simplified code of Bzip2 tests.
Remove compressed flag from HTTP/2

Motivation:

There are still a few places in the HTTP/2 code that have the compressed
flag (from pre-draft 13). Need to remove this flag since it's no longer
used.

Modifications:

Various changes to remove the flag from the writing path.

Result:

No references to the compressed flag.
Use value types for class and type in DNS entries to make them immune to parameter order bugs

Motivation:

DNS class and type were represented as integers rather than an enum or a
similar dedicated value type.  This can be a potential source of a
parameter order bug which might be difficult to track down.

Modifications:

Add DnsClass and DnsType to replace integer parameters

Result:

Type safety and less error-proneness
Overall cleanup of codec-dns

Related issue: #2688

- DnsClass and DnsType
  - Make DnsClass and DnsType implement Comparable
  - Optimize the message generation of IllegalArgumentException,
    by pre-populating the list of the expected parameters
  - Move the static methods up
  - Relax the validation rule of DnsClass so that a user can define a
    CLASS which is not listed in the RFC 1035
  - valueOf(int) does not throw IllegalArgumentException anymore as long
    as the specified value is an unsigned short.
  - Rename create() and forName() to valueOf() so that they look like a
    real enum
  - Rename type() and clazz() to intValue() so that they conform to our
    naming convention
- Add missing null checks in DnsEntry
Fix a compilation error
Updated the ChannelGroup JavaDoc by removing b.releaseExternalResources();
[#2705] Call fireChannelReadComplete() if channelActive(...) decodes messages in ReplayingDecoder / ByteToMessageDecoder

Motivation:

In ReplayingDecoder / ByteToMessageDecoder channelInactive(...) method we try to decode a last time and fire all decoded messages throw the pipeline before call ctx.fireChannelInactive(...). To keep the correct order of events we also need to call ctx.fireChannelReadComplete() if we read anything.

Modifications:

- Channel channelInactive(...) to call ctx.fireChannelReadComplete() if something was decoded
- Move out.recycle() to finally block

Result:

Correct order of events.
Fix compile error