Site Health: Detect an active PHP session as a possible reason for HTTP requests timing out.

PHP sessions created by a `session_start()` function call may interfere with REST API and loopback requests.

An active session should be closed by `session_write_close()` before making any HTTP requests.

Props matthieumota, netweblogic, Clorith, afragen, vjik, SergeyBiryukov.
Fixes #47320.
Built from https://develop.svn.wordpress.org/trunk@47585


git-svn-id: https://core.svn.wordpress.org/trunk@47360 1a063a9b-81f0-0310-95a4-ce76da25c4cd