* swarm/network: fix data race in stream.(*Peer).handleOfferedHashesMsg()

handleOfferedHashesMsg() contained a data race:
- read => in a goroutine, call to c.batchDone()
- write => in the main thread, write to c.sessionAt

c.batchDone() contained a call to c.AddInterval(). Client was a value
receiver for AddInterval. So on c.AddInterval() call the whole client
struct got copied (read) while one of its field was modified in
handleOfferedHashesMsg() (write).

fixes ethersphere/go-ethereum#1086

* swarm/network: simplify some trivial statements