Consolidate `HTTP` Clients

[johha]

Issue

cloud_controller_ng uses multiple HTTP clients:

Gem	used for	last updated
Net::HTTP	uaa_client_manager.rb service_gateway_client.rb	Ruby system package
httpclient	cdn.rb (fog) OPI Routing UAA Diego Credhub service_brokers	2016
em-http-request	async_requests.rb	2021

Goal

The same HTTP client should be used everywhere

Requirements

• Should receive regular updates
• Should handle DNS failovers, see:
  Diego::Client targets active bbs instance with id from locket #3002
  Diego client tries all bbs domain IPs #3048
  Use Net::HTTP::Persistent in Diego client #3170
  Patch HTTPClient to accept socket_connect_timeout #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 (Revert "Replace Timeout.timeout in Net:HTTP#connect" ruby/net-http#74)
• net-http-persistent: Extends Net::HTTP with thread safety and persistent connections
• excon
• httbrb