Replace UniqueName with Constant and ConstantPool

Proposed fix for #1824

`UniqueName` and its subtypes do not allow getting the previously registered instance.  For example, let's assume that a user is running his/her application in an OSGi container with Netty bundles and his server bundle.  Whenever the server bundle is reloaded, the server will try to create a new `AttributeKey` instance with the same name.  However, Netty bundles were not reloaded at all, so AttributeKey will complain that the name is taken already (by the previously loaded bundle.)

To fix this problem:

- Replaced `UniqueName` with `Constant`, `AbstractConstant`, and `ConstantPool`.  Better name and better design.

- `Sctp/Udt/RxtxChannelOption` is not a `ChannelOption` anymore.  They are just constant providers and `ChannelOption` is final now.  It's because caching anything that's from outside of netty-transport will lead to `ClassCastException` when `netty-transport-sctp/udt/rxtx` is reloaded, because `ChannelOption`'s constant pool will keep all option objects for reuse.

- `Signal` implements `Constant` because we can't ensure its uniqueness anymore by relying on the exception raised by `UniqueName`'s constructor.