BUG/MEDIUM: ssl: properly protect SSL cert generation

BUG/MEDIUM: ssl: properly protect SSL cert generation

Commit 821bb9b ("MAJOR: threads/ssl: Make SSL part thread-safe") added
insufficient locking to the cert lookup and generation code : it uses
lru64_lookup(), which will automatically remove and add a list element
to the LRU list. It cannot be simply read-locked.

A long-term improvement should consist in using a lockless mechanism
in lru64_lookup() to safely move the list element at the head. For now
let's simply use a write lock during the lookup. The effect will be
minimal since it's used only in conjunction with automatically generated
certificates, which are much more expensive and rarely used.

This fix must be backported to 1.8.