Handle unknown commands correctly

[zuiderkwast]

By default, we assume the key is the first argument after the command name, with known exceptions. It's not correct in some cases and it's not compatible with future Redis commands.

A better fallback would be to ask Redis about the keys in a command. It can be done using the COMMAND command (with logic for all commands that can be cached) or COMMAND GETKEYS for a specific command string, if the syntax is too complex.