Opentelemetry-java: Implement the ALWAYS_PARENT sampler

[jkwatson]

See https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#built-in-samplers in the specs

[Oberon00]

Spec needs to be updated to say what should happen if there is no parent (I think technically it says now that the sampled flag should be "false" then, but I wonder if that fits the name). Related https://github.com/open-telemetry/opentelemetry-specification/issues/492

[bogdandrutu]

I think the specs is clear, it says if it has a parent and parent is sampled then true otherwise false. I am not sure what is the confusion.

[bogdandrutu]

I think I got the confusion and opened a PR in specs.

[MitchellDumovic]

I think https://github.com/open-telemetry/opentelemetry-java/pull/1444 resolves this issue?

[jkwatson]

closing by #1444 .