merges stellar horizon-v0.15.4 with our changes

horizon: 0.16.0 CHANGELOG (stellar#851)

Horizon 0.15.4 CHANGELOG (stellar#818)

Horizon 0.15.3

Horizon 0.15.2

first production version of spam prevention implementation

Horizon 0.15.1

Horizon 0.15.0

SSE Pub/Sub feature, based on v0.12.3

improves Horizon database performance

Merge pull request stellar#710 from stellar/support-log-time-format

Change default time format to display miliseconds and full time in TTY