Notes:
- the module has to be started before any modules using the connections
- ‘ejabberd_auth_http’ and ‘mod_http_notification’ modules will use
  this module in the future.

To enable the module, add the corresponding config to ejabberd.cfg, eg.

{mod_http_client, [{pools, [{pool1, [{host, “http://host.com”},
	                        {pool_size, 100}]
                   }]
}

This should be added before any module using the pool.

To enable, set the following options in ejabberd.cfg

1. Enable ‘mod_http_client’, specifying a connection pool:

{mod_http_client, [{pools, [{muc_http_auth, [{host, "http://localhost:8080"},
                                             {path_prefix, "/muc/auth/"},
                                             {pool_size, 20}]}

The above config has to occur before the MUC config to provide the pool
when the MUC module starts.
For details, see the ‘mod_http_client’ module.

2. Use the pool in MUC config:

{mod_muc, [{host, "muc.@host@"},
           {access, muc},
           {access_create, muc_create},
           {http_auth_pool, muc_http_auth}]}

As a result, all rooms will:
- become password-protected by default
- call the external HTTP service instead of checking the configured
  password, whenever a new user enters the room

The external HTTP service has to respond with:
- code 200 and body ‘true’ when the password is accepted;
- code 200 and other body when the password is rejected - the body will
  be sent back to the entity in the XMPP error response in the <text>
  element
- other code when an error occurs, this will result in a
‘service-unavailable’ XMPP error

Also make overflow configurable and add tests for mod_http_client

Motivation:
* Tests using the same server configuration can be run in parallel,
* Adding basic features like parsing GET/POST requests
    was like reinventing the wheel.

This prevents race condition when another user would step in
  while the owner is still authenticating.

Motivation:
* In case of multiple hosts one pool per host is not efficient
* The module would need to be started first, complicating module deps
* Riak connectivity layer was not a module
* ejabberd.cfg is easier to understand without the extra module

(copy from small tests, maybe improve it later)