Make timestamps more generic objects

Timestamps should be more generic.

• Make an OperationUID interface, providing getClientUId(), compareTo() and (de)serialization methods.
• Make Timestamp implement OperationUID
• Use OperationUID & remove direct access to properties in VersionVector
• Use OperationUID & remove direct access to properties in PNCounter

To be discussed: the Timestamp.CNT_MIN_VALUE specific value is used in VersionVector as a default value for inexistent entries (in get() and max()). This must be replaced by either:

• null, which we tried to avoid as it adds complexity to comparisons
• an implementation-specific CNT_MIN_VALUE, to be provided by classes implementing OperationUID, that VersionVector constructor receives (directly or via a reference to an OperationUID object used as factory).
  Note that making VersionVector a generic type would not be enough, due to type erasure.

This could also lead to serialization issues.