Advanced Druid Client and Client API

[anskarl]

• Adds API for implementing custom clients
• Adds Advanced Druid client with cached host connection pool, load balancing and queue
• Updates configuration to support multiple Druid Brokers and custom configuration for Clients
• Adds health check per Broker
• Adds and updates tests
• Adds docker-compose with ClusterF images for unreliable HTTP proxy in order to simulate failures
• Updates CircleCI config to use docker-compose
• Adds tests for configuration overrides
• Adds documentation for Druid Clients and their API
• Adjusts timeouts in unit tests
• Adds a configuration builder for DruidAdvancedHttpClient, in order to programmatically define the parameters of the client

GH-61