Update HAT metadata link for new paper, remove Bolt-on

I'm flattered to have made the list, but I'd suggest two changes:
1.) Change the HAT paper to point to our extended VLDB paper instead of the HotOS workshop paper.
2.) Remove the link to Bolt-on Causal Consistency; this was a fun paper, but I don't think it deserves to be above, say, Lamport's paper and the other great papers here.
Add "Optimistic Replication" survey

Saito and Shapiro's survey is a top-notch overview of weakly consistent replication techniques and consistency models up to 2005.
Merge pull request #6 from pbailis/master

Update HAT metadata, add "Optimistic Replication" survey