url: close TLS before removing conn from cache

- Fix potential crashes in schannel shutdown.

Ensure any TLS shutdown messages are sent before removing the
association between the connection and the easy handle. Reverts
@bagder's previous partial fix for #3412.

Fixes https://github.com/curl/curl/issues/3412
Fixes https://github.com/curl/curl/issues/3505
Closes https://github.com/curl/curl/pull/3531