lib: TLS session ticket caching reworked

Described in detail in internal doc TLS-SESSIONS.md

Main points:
- use a new `ssl_peer_key` for cache lookups by connection filters
- recognize differences between TLSv1.3 and other tickets
  * TLSv1.3 tickets are single-use, cache can hold several of them for a peer
  * TLSv1.2 are reused, keep only a single one per peer
- differentiate between ticket BLOB to store (that could be persisted) and object instances
- use put/take/return pattern for cache access
- remember TLS version, ALPN protocol, time received and lifetime of ticket
- auto-expire tickets after their lifetime

Closes #15774

lib/share.h

@@ -31,6 +31,8 @@
 #include "urldata.h"
 #include "conncache.h"
 
+struct Curl_ssl_scache;
+
 #define CURL_GOOD_SHARE 0x7e117a1e
 #define GOOD_SHARE_HANDLE(x) ((x) && (x)->magic == CURL_GOOD_SHARE)
 
@@ -58,9 +60,7 @@ struct Curl_share {
   struct hsts *hsts;
 #endif
 #ifdef USE_SSL
-  struct Curl_ssl_session *sslsession;
-  size_t max_ssl_sessions;
-  long sessionage;
+  struct Curl_ssl_scache *ssl_scache;
 #endif
 };
 
@@ -68,4 +68,9 @@ CURLSHcode Curl_share_lock(struct Curl_easy *, curl_lock_data,
                            curl_lock_access);
 CURLSHcode Curl_share_unlock(struct Curl_easy *, curl_lock_data);
 
+/* convenience macro to check if this handle is using a shared SSL spool */
+#define CURL_SHARE_ssl_scache(data) (data->share &&                      \
+                                    (data->share->specifier &           \
+                                     (1<<CURL_LOCK_DATA_SSL_SESSION)))
+
 #endif /* HEADER_CURL_SHARE_H */