Bug 456968

The DNA plug-in currently shares a single mutex to protect the generation of new
values across all configured ranges.  While this approach is safe, it can cause
excessive thread contention if you have multiple managed ranges configured.
Let's take the scenario where you have two managed ranges configured. One range
is for the "uidNumber" attribute for "objectclass=posixAccount" entries, and the
other range is for the "gidNumber" attribute for "objectclass=posixGroup"
entries. With a single shared lock, a thread that is handling an operation to
add a new group would be blocked by another thread that is trying to add a new
entry. There is no need for these threads to block each other, as they will each
be using a new value from separate ranges.

A better approach would be to create a mutex as a part of each cached
configuration object. We have one cached configuration object per configured
range. When we load the configuration, we could create the mutexes and store
them with the rest of the configuration. With this approach, a thread would only
need to lock the range that it is getting a new value from, which would not
block threads working with other configured ranges.  The attached diffs
implement this approach.