easy: fix curl_easy_upkeep for shared connection caches

- Determine which connection cache is or will be used by the easy handle
  and perform connection upkeep on that cache.

This change allows curl_easy_upkeep to be effective on easy handles that
are using a shared connection cache, either from a user created shared
connection cache or a user created multi which has its own shared
connection cache.

Prior to this change curl_easy_upkeep would upkeep the connection cache
for the easy handle only if that cache was from the multi owned by the
easy handle (ie curl_easy_perform was previously called and there's a
connection cache exclusive to the easy handle in
data->multi_easy->conn_cache).

Ref: https://curl.se/mail/lib-2024-01/0016.html

Closes https://github.com/curl/curl/pull/12677

docs/libcurl/curl_easy_upkeep.md

@@ -39,6 +39,10 @@ This function must be explicitly called in order to perform the upkeep work.
 The connection upkeep interval is set with
 CURLOPT_UPKEEP_INTERVAL_MS(3).
 
+If you call this function on an easy handle that uses a shared connection cache
+then upkeep is performed on the connections in that cache, even if those
+connections were never used by the easy handle. (Added in 8.10.0)
+
 # %PROTOCOLS%
 
 # EXAMPLE