Fix pcapng timestamp resolution + imported-session duration

- pcap.py: _parse_tsresol now pads options to a 32-bit boundary
  ((-length) % 4) and uses 2** for the MSB-set (power-of-2) case, so
  nanosecond if_tsresol is read correctly (was defaulting to µs and
  scaling timestamps 1000x)
- database/server: set imported session started/ended from the first/last
  packet timestamps (falling back to wall-clock for synthetic SPB)
- frontend: formatDuration renders hours
- tests: tsresol (ns after padded option, default µs, ns round-trip) and
  session-window/fallback (119 total)

simtrace2_pysniff/pcap.py

@@ -378,9 +378,9 @@ def _parse_tsresol(options, endian):
         if code == 9 and length == 1:
             b = options[i]
             if b & 0x80:
-                return 10 ** (-(b & 0x7F))
+                return 2 ** (-(b & 0x7F))
             return 10 ** (-b)
-        i += length + (length & 1)  # options are padded to 32-bit
+        i += length + ((-length) % 4)  # options are padded to a 32-bit boundary
     return 1e-6  # default: microseconds