for OTP versions prior to 23, local function references in format like fun some_fun/2 behave in the same way as anonymous functions. different places in code generate different references. in addition to that local fun references are not code-reload safe, so it’s better to use external fun references only.

It's still a good idea to use function references, because it allows dialyzer to check the handler function specs.

…ew hooks used in testing

This branch introduces initial rework of the hooks framework

The key changes:
* hook handler functions now have fixed number of arguments:
   - Acc - the same meaning as before
   - Parameters map - input parameters are now provided in a form of map instead of variable number of handler function arguments
   - Extra map - every hook handler now can have a static context, in the same way as packet handlers. This map is automatically extended with 'hook_name' and 'hook_tag' ('global' or HostType) parameters on hook registration, so there is no need to add HostType into Parameters map any more.
* the old format of hook handlers is still supported (until the full conversion of all the hook handlers is done)
* conversion of the hooks can be done hook by hook, the following steps must be done to convert the hook:
   - convert hook execution function at 'mongoose_hooks' module.
   - convert all the handlers of the hook (they must be registered using gen_hook interface instead of ejabberd_hook).
   - also, it's possible to mix old & new hook handlers formats (see example at mongoose_hooks:push_notifications/4).
* note that 'gen_hook' accepts only external function references (in format "fun Module:Function/Arity', e.g. "fun erlang:is_function/1") as hook handler function parameter.