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.

src/cryptonote_core/blockchain.cpp

@@ -3313,6 +3313,11 @@ bool Blockchain::get_hard_fork_voting_info(uint8_t version, uint32_t &window, ui
   return m_hardfork->get_voting_info(version, window, votes, threshold, earliest_height, voting);
 }
 
+std::map<uint64_t, uint64_t> Blockchain:: get_output_histogram(const std::vector<uint64_t> &amounts) const
+{
+  return m_db->get_output_histogram(amounts);
+}
+
 void Blockchain::load_compiled_in_block_hashes()
 {
   if (m_fast_sync && get_blocks_dat_start(m_testnet) != nullptr)