tdb: allow reads after prepare commit

(Imported from SAMBA commit 46c99ec2a3781d53138245182345c6c2ddd9a258:
From: Andrew Tridgell <tridge@samba.org>
Date: Tue, 15 Sep 2009 14:04:22 -0700

We previously only allowed a commit to happen after a prepare
commit. It is in fact safe to allow reads between a prepare and a
commit, and the s4 replication code can make use of that, so allow it.