python/samba/samdb: Only do caching of well known DNs in dbcheck

python/samba/samdb: Only do caching of well known DNs in dbcheck

The fact that get_wellknown_dn() returned a cached DN that could
not be modified safely was unexpected, particularly given that
other similar routines did not do that.

The use case given at the time this was written by
Matthieu Patou in 6122acad0f1a7bc23b6f58862c16968e13da979d
was dbcheck, so move the cache there, and name it clearly.

dbcheck is the only case that uses this rotuine in an inner
loop.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jo Sutton <josutton@catalyst.net.nz>