v0.16.0: Improved logging, multiple minor crashes fixed, better libconsensus

Major improvements since v0.15.0:

• The /validators endpoint will now longer return a 500 when called with height=0;
• Ledger.getValidators: Return only validators that are not yet slashed;
• Remove an ambiguous usage of PublicKey (instead of UTXO) in signature aggregation;
• Key mapping have been removed from the EnrollmentManager, reducing memory usage;
• BitMask: Fixed serialization to not duplicate the length field, fixing a DoS vector;
• Admin endpoints now require the use of TLS (HTTPS);
• Admin endpoints now support basic auth, which can be configured from the config file;
• Some logging statements have been adjusted, resulting in less noise being produced at Info level;
• Validator nodes that cannot enroll, regardless of the reason, will no longer crash on shutdown;
• SCP logging will now produce human-readable strings for node ID;
• SCP logging is now fully configurable, including trace / debug level, like any other logging system;
• UTXODB will now always return ordered UTXO;
• Renamed endpoint stats for GET /block_height from block_heigth to block_height;