Finalize timestamp type

[asfimport]

support int64 UNIX timestamps with a particular resolution.

Parquet supports timestamp millis and micros (no nanos)
https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#datetime-types

currently Arrow timestamps have a timezone field.
https://github.com/apache/arrow/blob/master/format/Message.fbs#L67
proposal to change timestamp as follows:

• remove "timestamp" field and say it's UTC
• add unit field (MICROS | MILLIS)

Reporter: Julien Le Dem / @julienledem
Assignee: Julien Le Dem / @julienledem

Related issues:

• Arrow release 0.1 (blocks)
• [Format] Add time zone metadata to Timestamp type (relates to)

PRs and other links:

• PR

_{Note: This issue was originally created as ARROW-315. Please see the migration documentation for further details.}

[asfimport]

Julien Le Dem / @julienledem:
PR: #156

[asfimport]

Greg Rahn:
IMHO an important clarification with timestamp is to agree that while the serialization format may be based of a unix time/UTC offset, technically the value has no time zone information (e.g. timestamp without time zone). This is means that no matter what the client or server timezone is set to, the value remains the same – it behaves the same way write/read would work for a string representing a given timestamp.

[asfimport]

Julian Hyde:
[~grahn] +1

[asfimport]

Julien Le Dem / @julienledem:
Issue resolved by pull request 156
#156