Add {nis,schema-compat}-ignore-subtree (subtrees under which we ignore
contents and updates )and {nis,schema-compat}-restrict-subtree (subtrees
out of which we ignore contents and updates, if set) settings, and
default the former to "cn=tasks,cn=config".

This should avoid cases where we're looking through the ldbm backend for
entries which have a dangling reference to a newly-added task (which,
because it's in the DSE, means we acquire an ldbm lock after acquiring
our internal lock) while also updating a compat entry after its source
entry is modified (for example, by the memberOf plugin, which results in
us attempting to acquire our lock while the ldbm lock is already held).