multihandle: add an ssl_scache here

The TLS session cache is now held by the multi handle unless it is
shared, so that all easy handles within a multi handle get the benefit
of sharing the same, larger, cache.

The multi handle session cache size is set to 25, unless it is the
internal one used for the easy interface - which still uses only 3.

Closes #15982

lib/multiif.h

@@ -47,7 +47,8 @@ void Curl_multi_connchanged(struct Curl_multi *multi);
    socket, connection and dns hashes */
 struct Curl_multi *Curl_multi_handle(size_t hashsize,
                                      size_t chashsize,
-                                     size_t dnssize);
+                                     size_t dnssize,
+                                     size_t sesssize);
 
 /* the write bits start at bit 16 for the *getsock() bitmap */
 #define GETSOCK_WRITEBITSTART 16