Use the timestamp logical types for timestamp and clientTimestamp

[asnare]

Avro 1.8 introduced logical types, including support for timestamps. This was done in a way that allows for backwards compatibility; clients that don't understand logical types simply see the underlying primitive type.

In our case this would mean updating the relevant lines in the schema:

{ "name": "timestamp",       "type": { "type": "long", "logicalType": "timestamp-millis" }},
{ "name": "clientTimestamp", "type": { "type": "long", "logicalType": "timestamp-millis" }},

Unfortunately we can't just turn this on due to some issue in Avro's code generation. Specifically, enabling logical types makes the generated code depend on Joda-Time. There's an open issue (addressed by apache/avro#248) that will address this, but it seems to have lost momentum.

At this point I don't think the benefit of the cleaner types is justified by extra dependency on Joda-Time; hopefully it won't be too long before this is addressed.