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/Makefile.am

@@ -57,6 +57,7 @@ INTERNALDOCS =                                  \
  internals/README.md                            \
  internals/SPLAY.md                             \
  internals/STRPARSE.md                          \
+ internals/TLS-SESSIONS.md                      \
  internals/WEBSOCKET.md
 
 EXTRA_DIST =                                    \