Query historical speeds with timezone/daylight_saving offset

[wangyoucao577]

Ususally we'll query historical speeds by UTC time, but the data of historical speeds are local time. This issue is target to solve this issue: either convert UTC to localtime when querying or pre-offset the data in memory by timezone when loading.
The daylight_saving is more complex, which may have to be done when query.

This a subtask of #150.

[wangyoucao577]

OSM stores one timezone field with content tz database name (e.g., “Africa/Abidjan”).

• https://wiki.openstreetmap.org/wiki/Key%3Atimezone
• https://taginfo.openstreetmap.org/keys/timezone#values
• https://en.wikipedia.org/wiki/List_of_tz_database_time_zones

It’s a open standard and uses widely. Also, it’s very easy to handle by many different languages/libraries(https://en.wikipedia.org/wiki/Tz_database#Use_in_software_systems).
I have had a quick try on both Golang and Python, both of them can convert time to another timezone by one line.
Here’s some code snippet by Python:

>>> utc_dt.strftime(fmt)
'2006-03-26 21:34:59 UTC+0000'
>>> au_tz = timezone('Australia/Sydney')
>>> au_dt = utc_dt.astimezone(au_tz)
>>> au_dt.strftime(fmt)
'2006-03-27 08:34:59 AEDT+1100'

After internal discussion, unidb will provide the tz_database_name too to easy the converting, then caller don't need to handle the daylight saving details. This task will be postponed until the tz_database_name provided.

[wangyoucao577]

Since there's almost no timezone data in OSM(only hundreds in Global, which is too less), I think support it for OSM now is meaningless. I'd like to close this issue and move the unidb timezone support into internal development.
We may come up this issue again once OSM provides sufficient timezone information.