Make ComplexDateTimeField useable as a TTL index

[jimmyshen]

Currently, ComplexDateTimeField is not useable as a TTL index. Right now, this is stored as a simple string to enable lexicographic comparison but MongoDB itself won't be able to use it for a TTL index. I think changing the storage to an array will do the trick:

[ISODate, microsecond]  # zero out seconds/milliseconds in ISODate

This should sort properly and work with MongoDB TTL indexes per the documentation:

http://docs.mongodb.org/manual/core/index-ttl/

Backwards compatibility can be maintained by just converting data on egress to DB.

Thoughts? If everyone agrees this would be a good course, I can work on the PR.

[MRigal]

Sounds reasonable

[MRigal]

ping @jimmyshen

[jimmyshen]

Hey @MRigal -- I've been busy and have not had a chance to take a stab at this