Move redis plugin from credis to hiredis and also add custom commands

Hi,

This pull request move the redis plugin from the credis library (which is not maintained) to hiredis (which is also included in the most common linux distributions, include debian and ubuntu), and as a bonus, add custom commands, i.e. execute a redis command an get the result as a value to gather (for example use LLEN to get a queue size).

Custom commands has the restriction that returned value must be an integer and only one integer.