Output duration values as ISO-8601, not Cassandra "standard" notation

[tatu-at-datastax]

Currently API Tables handling of CQL duration type:

1. Accepts ISO-8601 duration formats (PnYnMnDTnHnMnS, like "PT1H30M", see https://en.wikipedia.org/wiki/ISO_8601) -- as well as Cassandra's "Standard" format (like "1h30m")
2. Produces Cassandra style values.

But specification defines that output should be done using ISO-8601 format.

[tatu-at-datastax]

Hmmmh. This is more complicated than I thought: turns out JDK 8 has TWO separate types, neither of which covers full range of CQL duration:

1. java.time.Duration: time-based amount of time (like "34.5 seconds") -- serialized like PT8H6M12.345S
2. java.time.Period: date-based amount of time (like "2 years, 3 months and 4 days") -- serialized like P6Y3M1D

So essentially JDK types are not good representations to cover duration type.