ITS#9536 Enable addition of targets to back-asyncmeta via cn=config and fix segfault

The patch enables the addition of a new asyncmeta database without a crash. Previously the existence of at least one target was assumed - now an asyncmeta database without targets can exist, although it will reject any LDAP requests. The patch also forbids the modification of any olcAsyncMetaConfig attributes if there is at least one operation waiting for a response. Additionally, attributes related to the remote connections cannot be modified if there are any established connections to the targets, and the number of target connections cannot be modified at all. Modification of olcDatabaseConfig attributes is allowed, however, olcSuffix should only be modified in order to "retire" the database in case of mis-configuration, as it will become inoperational.