Addings helper methods to HTTP/2 handler

Motivation:

Subclasses of AbstractHttp2ConnectionHandler have to implement all frame
handler methods, many of which can be ignored in many cases.  Also there
is no easy way to access the connection object.

Modifications:

Added default implementations for frame handler methods to
AbstractHttp2ConnectionHandler, and added an accessor for the
connection.

Also fixed example test for HTTP/2 with cleartext upgrade. It must have
been broken by recent commits.

Result:

AbstractHttp2ConnectionHandler is more subclass-friendly.
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