* swarm/newtork: WIP Span request span until delivery and put

* swarm/storage: Introduce new trace across single fetcher lifespan

* swarm/network: Put span ids for sendpriority in context value

* swarm: Add global span store in tracing

* swarm/tracing: Add context key constants

* swarm/tracing: Add comments

* swarm/storage: Remove redundant fix for filestore

* swarm/tracing: Elaborate constants comments

* swarm/network, swarm/storage, swarm:tracing: Minor cleanup