Remove 'iqdisc' option

Remove 'iqdisc' option

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