Inserting values failed when timestamp is in future and TTL is calculated
Description
Inserting values with a future timestamp fails with an Exception if calculateTimeToLive is set to ```true```.
The following exception is raied: ``` java.lang.IllegalArgumentException: you can only subtract an earlier date from a later one... negative durations don't make sense at org.opennms.newts.api.Timestamp.minus(Timestamp.java:77) ~[newts-api-1.3.3-SNAPSHOT.jar:?] at org.opennms.newts.persistence.cassandra.CassandraSampleRepository.insert(CassandraSampleRepository.java:245) ~[newts-cassandra-1.3.3-SNAPSHOT.jar:?] ```
This also happens if the timestamp is equal to the current one.
Inserting values with a future timestamp fails with an Exception if calculateTimeToLive is set to ```true```.
The following exception is raied:
```
java.lang.IllegalArgumentException: you can only subtract an earlier date from a later one... negative durations don't make sense
at org.opennms.newts.api.Timestamp.minus(Timestamp.java:77) ~[newts-api-1.3.3-SNAPSHOT.jar:?]
at org.opennms.newts.persistence.cassandra.CassandraSampleRepository.insert(CassandraSampleRepository.java:245) ~[newts-cassandra-1.3.3-SNAPSHOT.jar:?]
```
This also happens if the timestamp is equal to the current one.