DateTime handling

ClickHouse.Client tries to correctly handle timezones and DateTime.Kind property. Specifically:

• DateTime values are returned as UTC. User can then convert them themselves or use ToLocalTime() method on DateTime instance
• When inserting, DateTime values are handled in following way:
  • UTC DateTimes are inserted 'as is', because ClickHouse stores them in UTC internally
  • Local DateTimes are converted to UTC according to user's local timezone settings
  • Unspecified DateTimes are considered to be in target column's timezone, and hence are converted to UTC according to that timezone
• For columns without DateTime specified, client timezone is used by default (legacy behavior). UseServerTimezone flag in connection string can be used to use server timezone