Change DTN timestamp to milliseconds

The internal representation of a DTN timestamp is changed form seconds to milliseconds according to an updated version of the bundle protocol specification. All data structures representing a DTN timestamp are affected by this change, including both uD3TN and the BPv7 Python library. This includes the creation timestamp, the lifetime and also extension blocks like the bundle age.

Since BPv6 still uses seconds, a conversion from milliseconds to seconds is performed during de-/serialization.

Only the timestamps of the bundle protocol implementation are changed. Other components, such as contact manager and CLAs, are not affected by the changes.

Fixes: #34 (closed)