* 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
