cpool: rename "connection cache/conncache" to "Connection Pools/cpool"

This is a better match for what they do and the general "cpool"
var/function prefix works well.

The pool now handles very long hostnames correctly.

The following changes have been made:

* 'struct connectdata', e.g. connections, keep new members
  named `destination` and ' destination_len' that fully specifies
  interface+port+hostname of where the connection is going to.
  This is used in the pool for "bundling" of connections with
  the same destination. There is no limit on the length any more.
* Locking: all locks are done inside conncache.c when calling
  into the pool and released on return. This eliminates hazards
  of the callers keeping track.
* 'struct connectbundle' is now internal to the pool. It is no
  longer referenced by a connection.
* 'bundle->multiuse' no longer exists. HTTP/2 and 3 and TLS filters
  no longer need to set it. Instead, the multi checks on leaving
  MSTATE_CONNECT or MSTATE_CONNECTING if the connection is now
  multiplexed and new, e.g. not conn->bits.reuse. In that case
  the processing of pending handles is triggered.
* The pool's init is provided with a callback to invoke on all
  connections being discarded. This allows the cleanups in
  `Curl_disconnect` to run, wherever it is decided to retire
  a connection.
* Several pool operations can now be fully done with one call.
  Pruning dead connections, upkeep and checks on pool limits
  can now directly discard connections and need no longer return
  those to the caller for doing that (as we have now the callback
  described above).
* Finding a connection for reuse is now done via `Curl_cpool_find()`
  and the caller provides callbacks to evaluate the connection
  candidates.
* The 'Curl_cpool_check_limits()' now directly uses the max values
  that may be set in the transfer's multi. No need to pass them
  around. Curl_multi_max_host_connections() and
  Curl_multi_max_total_connections() are gone.
* Add method 'Curl_node_llist()' to get the llist a node is in.
  Used in cpool to verify connection are indeed in the list (or
  not in any list) as they need to.

I left the conncache.[ch] as is for now and also did not touch the
documentation. If we update that outside the feature window, we can
do this in a separate PR.

Multi-thread safety is not achieved by this PR, but since more details
on how pools operate are now "internal" it is a better starting
point to go for this in the future.

Closes #14662

lib/multiif.h

@@ -63,15 +63,6 @@ struct Curl_multi *Curl_multi_handle(size_t hashsize,
 /* mask for checking if read and/or write is set for index x */
 #define GETSOCK_MASK_RW(x) (GETSOCK_READSOCK(x)|GETSOCK_WRITESOCK(x))
 
-/* Return the value of the CURLMOPT_MAX_HOST_CONNECTIONS option */
-size_t Curl_multi_max_host_connections(struct Curl_multi *multi);
-
-/* Return the value of the CURLMOPT_MAX_TOTAL_CONNECTIONS option */
-size_t Curl_multi_max_total_connections(struct Curl_multi *multi);
-
-void Curl_multiuse_state(struct Curl_easy *data,
-                         int bundlestate); /* use BUNDLE_* defines */
-
 /*
  * Curl_multi_closed()
  *