rare assertion failure on test054-syncrepl-parallel-load on a busy machine

[erthink]

160606-23:08:16.942842_09506 *** Backtrace by addr2line:
160606-23:08:17.047166_09506 (0) __ldap_assert_fail у /home/ly/Projects/reopenldap.git/@ci-buzz.pool/@4.devel/src/servers/slapd/globals.c:245
160606-23:08:17.065891_09506 (1) syncprov_sendresp у /home/ly/Projects/reopenldap.git/@ci-buzz.pool/@4.devel/src/servers/slapd/overlays/syncprov.c:1139
160606-23:08:17.065929_09506 (2)  (внутристрочный) syncprov_playback_locked у /home/ly/Projects/reopenldap.git/@ci-buzz.pool/@4.devel/src/servers/slapd/overlays/syncprov.c:1178
160606-23:08:17.065943_09506 (3)  (внутристрочный) syncprov_playback_dequeue у /home/ly/Projects/reopenldap.git/@ci-buzz.pool/@4.devel/src/servers/slapd/overlays/syncprov.c:1235
160606-23:08:17.072732_09506 (4) ldap_int_thread_pool_wrapper у /home/ly/Projects/reopenldap.git/@ci-buzz.pool/@4.devel/src/libraries/libreldap/tpool.c:721
slapd: syncprov.c:1128: syncprov_sendresp: Проверочное утверждение «ldap: a && slap_csn_match( &ri->ri_csn, a->a_nvals )» не выполнено.

So, syncprov tries to send a LDAP_RES_SEARCH_ENTRY message with a cookie, but the cookie.contextCSN != entryCSN.

[erthink]

Assuming this will be fixed just by biglock, but testing needed.