add cancel strategies (ITS#4560)

servers/slapd/back-ldap/back-ldap.h

servers/slapd/back-ldap/bind.c

servers/slapd/back-ldap/config.c

servers/slapd/back-ldap/init.c

servers/slapd/back-ldap/proto-ldap.h

servers/slapd/back-ldap/search.c

servers/slapd/back-meta/add.c

servers/slapd/back-meta/back-meta.h

servers/slapd/back-meta/bind.c

servers/slapd/back-meta/config.c

servers/slapd/back-meta/delete.c

servers/slapd/back-meta/init.c

servers/slapd/back-meta/map.c

servers/slapd/back-meta/modify.c

servers/slapd/back-meta/modrdn.c

servers/slapd/back-meta/search.c