New RPC and daemon command to get output histogram

This is a list of existing output amounts along with the number
of outputs of that amount in the blockchain.

The daemon command takes:
- no parameters: all outputs with at least 3 instances
- one parameter: all outputs with at least that many instances
- two parameters: all outputs within that many instances

The default starts at 3 to avoid massive spamming of all dust
outputs in the blockchain, and is the current minimum mixin
requirement.

An optional vector of amounts may be passed, to request
histogram only for those outputs.

tests/unit_tests/hardfork.cpp

@@ -108,6 +108,7 @@ public:
   virtual bool for_all_transactions(std::function<bool(const crypto::hash&, const cryptonote::transaction&)>) const { return true; }
   virtual bool for_all_outputs(std::function<bool(uint64_t amount, const crypto::hash &tx_hash, size_t tx_idx)> f) const { return true; }
   virtual bool is_read_only() const { return false; }
+  virtual std::map<uint64_t, uint64_t> get_output_histogram() const { return std::map<uint64_t, uint64_t>(); }
 
   virtual void add_block( const block& blk
                         , const size_t& block_size