Allow `record.last_modified` to be any string?

[michielbdejong]

Currently we're assuming that the ETag of a record is numerical. Can we drop that restriction? It's stopping me from using kinto.js with ludbud (a remoteStorage client) instead of kinto-client.

[leplatrem]

Yes, +1

Note: See also #356

[n1k0]

This change is definitely required to comply with the Kinto protocol, and is a major requirement for 2.0.

[leplatrem]

Note that this would require a sqlite schema migration in firefox storage \o/