Always use mcd:mcdkey/1 to convert terms to keys

Rename mcd:md5 to mcdkey to open the possibility for different
conversions, namely NO conversion.
Support application env raw_keys option

When raw_keys is set to true, no conversion of keys is performed,
all keys need to conform to memcached restrictions.
When raw_keys is set to false or not set, the implicit erlang term
and MD5 conversion is applied.