Fix crash in destructor when using libcxx (Clang STL, FreeBSD)

This code crashes in the destructor of LdapClient on FreeBSD.
The crash is visible to uses who
- start KOrganizer
- click the "New Event" button

The underlying issue is a lot like the one described in
	https://blogs.kde.org/2021/02/20/uniqueptr-difference-between-libstdc-and-libc-crashes-your-application
There is a convoluted call-chain on destruction of LdapClient:
- ~LdapClient
- ~std::unique_ptr<LdapClientPrivate>
- ~LdapClientPrivate
- LdapClient::cancelQuery
- (accesses to members of LdapClientPrivate `d`)

With libcxx, the pointer in `d` is already set to nullptr and
SEGV happens. I'm not sure it isn't UB, anyway, since the
destructor body for LdapClient has already run.

The fix moves the implementation of `cancelQuery()` into
the private class. This means that the LdapClient class does
a little less poking-and-prodding in the private class,
but also cuts out the call-from-private-back-to-destroyed-
owning-LdapClient, fixing the SEGV and possible UB.