conncache: use conn->data to know if a transfer owns it

- make sure an already "owned" connection isn't returned unless
  multiplexed.

- clear ->data when returning the connection to the cache again

Regression since 7.62.0 (probably in commit 1b76c389)

Bug: https://curl.haxx.se/mail/lib-2019-03/0064.html

Closes #3686