Add a case-insensitive option directly to siphash 2-4 implementation

Add a case-insensitive option directly to siphash 2-4 implementation

Formerly, the isc_hash32() would have to change the key in a local copy
to make it case insensitive.  Change the isc_siphash24() and
isc_halfsiphash24() functions to lowercase the input directly when
reading it from the memory and converting the uint8_t * array to
64-bit (respectively 32-bit numbers).