add an option to deal with couchdb conflicts

[tdebarochez]

In the case where two concurrent commits are made on the same session, a conflict might occurs. I see three ways to deals with it :

• throw an exception (actual scenario, but it should be better handled)
• ignore it
• erase the actual stored session

[eddiesholl]

Any interest in this issue getting addressed? I can see how different users would want to either ignore or throw, based on how important their session data is. Consider a PR?

[tdebarochez]

I gladly accept every well tested PRs. :)

[eddiesholl]

I've added basic support for ignoring conflicts at https://github.com/eddiesholl/connect-couchdb/tree/conflicts, I'm getting a few other test breakages that I don't think are caused by my changes. If you have a moment do you mind trying it out?

Do you think support for passing in a conflict handling function is required here as well?