schema: Do not read different schema sequence values during a read transaction

schema: Do not read different schema sequence values during a read transaction

During a read lock, we find ourselves seeing an unchanged schema, but
reading any updates to the metadata.tdb (in the case of lmdb, where
reads do not block writes).

The alternative is to read-lock the entire metadata.tdb, however, this
allows more concurrency by allowing reads not to block writes.

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>