avoid holding onto temporary buffers in SpdyFrameCodec

Conflicts:
	src/main/java/org/jboss/netty/handler/codec/spdy/SpdyHeaderBlockRawDecoder.java
	src/main/java/org/jboss/netty/handler/codec/spdy/SpdyHeaderBlockZlibDecoder.java
Merge branch '3' into netty3spdybuf

Conflicts:
	src/main/java/org/jboss/netty/handler/codec/spdy/SpdyHeaderBlockZlibDecoder.java
Merge pull request #1929 from wgallagher/netty3spdybuf

avoid holding onto temporary buffers in SpdyFrameCodec
Backport HttpHeaders / SpdyHeaders from Netty 4.