Consolidate `HTTP` Clients

Issue

cloud_controller_ng uses multiple HTTP clients:

Goal

The same HTTP client should be used everywhere

Requirements

• Should receive regular updates
• Should handle DNS failovers, see:
  #3002
  #3048
  #3170
  #3174
  Can be tested locally with https://github.com/johha/dns-failover-test
• Should be thread safe
• Should allow persistent connections (nice to have)

Some Suitable HTTP Clients

• Net::HTTP: Default Ruby HTTP client which could be sufficient as soon as connect_timeout is correctly implemented in TCPSocket (https://github.com/ruby/net-http/pull/74)
• net-http-persistent: Extends Net::HTTP with thread safety and persistent connections
• excon
• httbrb

Start by inventorying the HTTP usage in lib/services/sso/uaa/uaa_client_manager.rb, lib/vcap/services/api/clients/service_gateway_client.rb, cdn.rb, async_requests.rb, and the named OPI, Routing, UAA, Diego, Credhub, and service_brokers entry points. Compare candidate clients against thread safety, DNS failover, timeout, and connection requirements, using the dns-failover-test project and issues #3002, #3048, #3170, and #3174. Done means the project uses one suitable client everywhere and the listed requirements are verified.