BUG/MEDIUM: ssl: Fix crash when calling "update ssl ocsp-response" when an update...

BUG/MEDIUM: ssl: Fix crash when calling "update ssl ocsp-response" when an update is ongoing

The CLI command "update ssl ocsp-response" was forcefully removing an
OCSP response from the update tree regardless of whether it used to be
in it beforehand or not. But since the main OCSP upate task works by
removing the entry being currently updated from the update tree and then
reinserting it when the update process is over, it meant that in the CLI
command code we were modifying a structure that was already being used.

These concurrent accesses were not properly locked on the "regular"
update case because it was assumed that once an entry was removed from
the update tree, the update task was the only one able to work on it.

Rather than locking the whole update process, an "updating" flag was
added to the certificate_ocsp in order to prevent the "update ssl
ocsp-response" command from trying to update a response already being
updated.

An easy way to reproduce this crash was to perform two "simultaneous"
calls to "update ssl ocsp-response" on the same certificate. It would
then crash on an eb64_delete call in the main ocsp update task function.

This patch can be backported up to 2.8.