Add precision to datetime field and datetime field internally expressed in microsecs.

[fulmicoton]

In order to have a better DateTime support , In Tantivy, we need to implement:

• Update the current DateTime type to hold unix-timestamp in microseconds
• The input parsing with support only for rfc3339 format.
• The query parsing with support only for rfc3339 format.
• DateTime fields will be output in unix-timestamp microsecond, This helps Tantivy users format the value in whatever format for display.
• Assume all DateTime received to be in UTC if not explicitly specified in the format

The DateTime field schema spec needs precision.

• FastField: We will internally store all dates in i64 using the unix-timestamp microseconds value. The precision option will be used to store what’s exactly needed in fast fields. As of now, we won’t support nanosecond precision.
• In docStore: We store the timestamp as i64 representing the unix-timestamp value in microseconds of the parsed input without any precision knowledge.