Since we got rid of all bottle-neck processes and we have
a connection pool for every database, the option is no longer
needed and in fact is detrimental: in practice what you get
is just a bunch of overloaded processes in the IQ handlers pool
no matter how much you increase the `iqdisc` value.
Given that there are close to zero operators understanding
the meaning of the option and, hence, not using it all,
it's not simply deprecated but completely removed.
The commit also deprecates the following functions:
- gen_iq_handler:add_iq_handler/6
- gen_iq_handler:handle/5
- gen_iq_handler:iqdisc/1
The changes are very similar to those from previous commit:
* Now there is no need to pass validating function in
gen_mod:get_opt() and gen_mod:get_module_opt() functions,
because the modules' configuration keeps already validated values.
* New functions gen_mod:get_opt/2 and gen_mod:get_module_opt/3 are
introduced.
* Functions gen_mod:get_opt/4 and get_module_opt/5 are deprecated.
If the functions are still called, the "function" argument is
simply ignored.
* Validating callback Mod:listen_opt_type/1 is introduced to validate
listening options at startup.
