This commit changes what provided by PR #3315 (merged) in order to
let the user specify the log level as a string.

The define could be also used, but when this happens, they must be
decoupled from the defines in the Redis core, like in the other part of
the Redis modules implementations, so that a switch statement (or a
function) remaps between the two, otherwise we are no longer free to
change the internal Redis defines.