codec-memcache: fix decoding issue for value parts.

This changeset fixes an issue when decoding would stop if the value
comes separated in two or more packets, leading to invalid bytes read.
codec-memcache: removed println in test case.
[codec-memcache] encode key directly, no need to use unpooled.
Add FixedCompositeByteBuf which can be used to write an array of ByteBuf in an efficient way.

This implementation does not produce as much GC pressure as CompositeByteBuf and so is prefered,
for writing an array of ByteBufs. Be aware that FixedCompositeByteBuf is readonly.

When using this in a project that make heavy use of CompositeByteBuf for writes we was able to cut
down allocation to a half.
Allow specifying a custom charset for a multipart Attribute

- Fixed #2025
- Adapted from @BabyDuncan's pull request: #2031
- Overall ugliness clean-up