dawn: update to latest version 2020-07-12

Includes:
- dawn_uci: no need uci_alloc_context on each uci_set_network call
- test_storage: extend test harness; datastorage: two bug fixes
- tcpsocket: free con after ustream write error and make list each safe
  list_each is not safe against removal of list entry

Signed-off-by: Nick Hainke <vincent@systemli.org>