scoreboard/status: follow up to r1739008.

Restore (completely) pre-2.4.20 behaviour w.r.t. preserved values.
r1739008 was still unnessessarily blanking some values for the time of
BUSY_READ -> BUSY_WRITE (with blocking MPMs).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1739146 13f79535-47bb-0310-9956-ffa450edef68