Opentelemetry-js: Update the OT Shim layer

[carlosalberto]

The OT Shim layer needs to be updated to include the latest changes as introduced by

https://github.com/open-telemetry/opentelemetry-specification/pull/1101
https://github.com/open-telemetry/opentelemetry-specification/pull/1563

[vmarchaud]

I'm not sure with this is required for GA cc @dyladan @obecny

[carlosalberto]

@vmarchaud It's not required for 1.0, for sure (as it's _still_ marked as Experimental).

[vreynolds]

Chunking up the updates:

• add shim constructor with custom propagators [done]
• spanShim.setTag updates
• spanShim.log updates
• span context immutability and managing association with span

I can work on the setTag updates next, will not claiming anything else at the moment :)