Get rid of andand.

Sometimes it's better to be explicit instead of implicit and avoiding
small hacks like andand.
Return a syntax error if input is not as expected.

Invalid syntax was parsed and only the relevant parts added as options
to the redis call. This is not expected and not how redis-cli works (we
try to act the same as redis-cli where possible)
Rename namespace_input → parse_command.

There's only one case left where we explicitely namespace it ourself.
Most of the time we're just parsing the input and transforming it into
redis-rb expected format.