conncache: lowercase the hash key for better match

As host names are case insensitive, the use of case sensitive hashing
caused unnecesary cache misses and therefore lost performance. This
lowercases the hash key.

Reported-by: Harry Sintonen
Fixes #7159
Closes #7161

lib/conncache.c

@@ -34,6 +34,7 @@
 #include "share.h"
 #include "sigpipe.h"
 #include "connect.h"
+#include "strcase.h"
 
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
@@ -161,6 +162,7 @@ static void hashkey(struct connectdata *conn, char *buf,
 
   /* put the number first so that the hostname gets cut off if too long */
   msnprintf(buf, len, "%ld%s", port, hostname);
+  Curl_strntolower(buf, buf, len);
 }
 
 /* Returns number of connections currently held in the connection cache.