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/urldata.h

@@ -271,21 +271,7 @@ enum protection_level {
 
 /* SSL backend-specific data; declared differently by each SSL backend */
 struct ssl_backend_data;
-
-typedef enum {
-  CURL_SSL_PEER_DNS,
-  CURL_SSL_PEER_IPV4,
-  CURL_SSL_PEER_IPV6
-} ssl_peer_type;
-
-struct ssl_peer {
-  char *hostname;        /* hostname for verification */
-  char *dispname;        /* display version of hostname */
-  char *sni;             /* SNI version of hostname or NULL if not usable */
-  ssl_peer_type type;    /* type of the peer information */
-  int port;              /* port we are talking to */
-  int transport;         /* one of TRNSPRT_* defines */
-};
+struct Curl_ssl_scache_entry;
 
 struct ssl_primary_config {
   char *CApath;          /* certificate dir (does not work on Windows) */
@@ -341,24 +327,6 @@ struct ssl_general_config {
   int ca_cache_timeout;  /* Certificate store cache timeout (seconds) */
 };
 
-typedef void Curl_ssl_sessionid_dtor(void *sessionid, size_t idsize);
-
-/* information stored about one single SSL session */
-struct Curl_ssl_session {
-  char *name;       /* hostname for which this ID was used */
-  char *conn_to_host; /* hostname for the connection (may be NULL) */
-  const char *scheme; /* protocol scheme used */
-  char *alpn;         /* APLN TLS negotiated protocol string */
-  void *sessionid;  /* as returned from the SSL layer */
-  size_t idsize;    /* if known, otherwise 0 */
-  Curl_ssl_sessionid_dtor *sessionid_free; /* free `sessionid` callback */
-  long age;         /* just a number, the higher the more recent */
-  int remote_port;  /* remote port */
-  int conn_to_port; /* remote port for the connection (may be -1) */
-  int transport;    /* TCP or QUIC */
-  struct ssl_primary_config ssl_config; /* setup for this session */
-};
-
 #ifdef USE_WINDOWS_SSPI
 #include "curl_sspi.h"
 #endif
@@ -1232,8 +1200,7 @@ struct UrlState {
   curl_prot_t first_remote_protocol;
 
   int retrycount; /* number of retries on a new connection */
-  struct Curl_ssl_session *session; /* array of 'max_ssl_sessions' size */
-  long sessionage;                  /* number of the most recent session */
+  struct Curl_ssl_scache *ssl_scache; /* TLS session pool */
   int os_errno;  /* filled in with errno whenever an error occurs */
   long followlocation; /* redirect counter */
   int requests; /* request counter: redirects + authentication retakes */