fix(networkstream): fix timeout, mutex stall, and empty-stream skip (#817)

- Increase default HTTP client timeout from 5s to 30s to match the
  synchronizer's new ReadTimeout and avoid racing timeouts causing
  spurious "context deadline exceeded" errors
- Release eventsStorageMutex before the HTTP send by snapshotting the
  current state first; the clearing loop never mutates Inbound/Outbound
  maps in place so a new Entities map with entity structs copied by value
  is sufficient for safe concurrent access
- Add missing return nil in sendNetworkEvent when the stream is empty so
  empty intervals are actually skipped instead of sending empty payloads

Signed-off-by: Matthias Bertschy <matthias.bertschy@gmail.com>