[#1906] Use a ByteBuf allocator from the ByteBufAllocator when encode Strings
Ensure the selector implementation can be instrumented before attempting instrumentation

- Fixes #1908
[#1907] LengthFieldPrepender should better extend MessageToMessageEncoder for less memory copies
minor gc optimization: better DefaultHttpHeaders.iterator()
minor gc optimization: better DefaultSpdyHeaders.iterator()
Add HttpHeaders.contains(name, value, ...) to save creation of LinkedList for simple checks
bring back entries()
Merge pull request #1918 from wgallagher/entries

bring back entries()
Fix compile error
copy all sliced buffers, fixes #1848
No need to slice before copy. Part of #1848