ITS#6262 regression

While running valgrind --tool=helgrind on test008 I saw some data races that were quite unexpected, in ldap_pvt_gettime(). These functions are supposed to detect duplicate timestamps being returned, as noted in git rev 6cbf65642a9d094feddf820b206c1a3af70d5fd9 .

This detection relies on static variables, and as such their invocations must be mutex-protected, as noted in the function comments (added immediately after, in ee2001ea4b26b0957040ca0753a94f26f78e6147).

Unfortunately in f3cdcadf89474e7267d9f8f17d2bac46551b2131 the necessary mutex was removed. This means on multi-processor machines it's possible for us to generate non-unique CSNs.