Switch from MurmurHash3 to SipHash to prevent DoS collision attack (hash flooding)

Thanks, we've talked about this issue and some possible things we could do, but didn't have a ticket for it yet.

While the Java 7 approach relied on (attempting) to properly seed hash maps with string hash codes, that was all dropped in Java 8, which addressed DoS collision hash attacks by instead improving the data structure to switch from linear collisions to a red/black tree (log-time) for collisions. It's possible a similar approach could work in Clojure as well.

One workaround that could be used now is to wrap map keys in a custom type that implements IHashEq and implements an alternate hash function.