Introduce FastThreadLocal which uses an EnumMap and a predefined fixed set of possible thread locals

Motivation:
Provide a faster ThreadLocal implementation

Modification:
Add a "FastThreadLocal" which uses an EnumMap and a predefined fixed set of possible thread locals (all of the static instances created by netty) that is around 10-20% faster than standard ThreadLocal in my benchmarks (and can be seen having an effect in the direct PooledByteBufAllocator benchmark that uses the DEFAULT ByteBufAllocator which uses this FastThreadLocal, as opposed to normal instantiations that do not, and in the new RecyclableArrayList benchmark);

Result:
Improved performance
Introduce TextHeaders and AsciiString

Motivation:

We have quite a bit of code duplication between HTTP/1, HTTP/2, SPDY,
and STOMP codec, because they all have a notion of 'headers', which is a
multimap of string names and values.

Modifications:

- Add TextHeaders and its default implementation
- Add AsciiString to replace HttpHeaderEntity
  - Borrowed some portion from Apache Harmony's java.lang.String.
- Reimplement HttpHeaders, SpdyHeaders, and StompHeaders using
  TextHeaders
- Add AsciiHeadersEncoder to reuse the encoding a TextHeaders
  - Used a dedicated encoder for HTTP headers for better performance
    though
- Remove shortcut methods in SpdyHeaders
- Replace SpdyHeaders.getStatus() with HttpResponseStatus.parseLine()

Result:

- Removed quite a bit of code duplication in the header implementations.
- Slightly better performance thanks to improved header validation and
  hash code calculation