Use GCS custom_time as timestamp metadata storage

[thclark]

Feature request

Use Case

GCS Allows a custom_time field to be associated with an object. If a non-null timestamp is provided to the Datafile, that is currently stored (as a posix timestamp) in the metadata, as a timestamp field.

However, using the GCS built in (fixed key) metadata to store this value, instead of our custom field, would allow us to more effectively filter timeseries files in the cloud store and elsewhere. It'd also allow us to encapsulate a relevant timezone where necessary.

Note that the storage client can accept timezoned datetime.datetime objects and they're serialized using RFC3339 as shown here which is a useful trick for us to use in future then we need to serialize datetimes.

[thclark]

NOPE!

We have to undo this.

I've just discovered that GCS' custom_time metadata is dumb as fuck.

So you can't change metadata that's incorrect.

we'll need to store in our own timestamp field after all, sorry @cortadocodes :(

[cortadocodes]

Reverted here: #160