Allow `record.last_modified` to be any string?

Currently we're [assuming that the ETag of a record is numerical](https://github.com/Kinto/kinto.js/blob/27a7644f6dd60e975bd8b33dad44fc5b16f34ffb/src/collection.js#L728). Can we drop that restriction? It's stopping me from using kinto.js with ludbud (a remoteStorage client) instead of kinto-client.

Start at src/collection.js around line 728, where the issue identifies the numerical ETag assumption, and trace how record.last_modified is handled. Done means record.last_modified can contain an arbitrary string so kinto.js can work with the mentioned remoteStorage client without the current restriction.

Written by the indexing model from the issue text.