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

docs/internals/LLIST.md

@@ -64,7 +64,13 @@ See also `Curl_llist_insert_next`.
 
 ## Remove a node
 
-Remove a node again from a list by calling `Curl_llist_remove()`.
+Remove a node again from a list by calling `Curl_llist_remove()`. This
+will destroy the node's `elem` (e.g. calling a registered free function).
+
+To remove a node without destroying it's `elem`, use
+`Curl_node_take_elem()` which returns the `elem` pointer and
+removes the node from the list. The caller then owns this pointer
+and has to take care of it.
 
 ## Iterate