The `otOperationalDataset#mActiveTimestamp` should represent the full
Active Timestamp TLV value but not only the seconds. The same for
`otOperationalDataset#mPendingTimestamp`.
This commit adds a new common `Timestamp::Compare()` methods which
compares two given `Timestamp` pointers (determines whether they are
equal or one is ahead or behind the other one). The new method allows
either input (or even both) to be `nullptr`. A null timestamp is used
to indicate when there is no timestamp. In terms of comparison
result, a non-null timestamp is considered to be ahead of (or larger
then) a null one and two null timestamps are considered equal. The
new method is used in all different places where the `Timestamp`s are
compared.
This commit also adds a unit test to verify the `Timestamp` class (the
existing `test_steering_data` is renamed to `test_meshcop` which is now
used to include unit tests for all MeshCoP related definitions).
Kangping