we can't use the unique index code for samAccountName

we can't use the unique index code for samAccountName

Using ldb unique indexes for samAccountName doesn't work with DRS as
the other DC may send us a deleted record (tombstone record), which
has the same samAccountName as an existing record. That would then
create two records in the same partition with the same samAccountName.

So we needed to put back the logic in samldb.c which explicitly
checked whether a samAccountName already exists on add