encrypted payment ids are now 64 bit, instead of 256 bit

Pros:
 - smaller on the blockchain
 - shorter integrated addresses

Cons:
 - less sparseness
 - less ability to embed actual information

The boolean argument to encrypt payment ids is now gone from the
RPC calls, since the decision is made based on the length of the
payment id passed.

src/cryptonote_core/cryptonote_basic.h

@@ -57,6 +57,7 @@ namespace cryptonote
 {
 
   const static crypto::hash null_hash = AUTO_VAL_INIT(null_hash);
+  const static crypto::hash8 null_hash8 = AUTO_VAL_INIT(null_hash8);
   const static crypto::public_key null_pkey = AUTO_VAL_INIT(null_pkey);
 
   typedef std::vector<crypto::signature> ring_signature;