r10421: following on discussions with simo, I have worked out a way of

allowing searches to proceed while another process is in a
transaction, then only upgrading the transaction lock to a write lock
on commit.

The solution is:

 - split tdb_traverse() into two calls, called tdb_traverse() and
   tdb_traverse_read(). The _read() version only gets read locks, and
   will fail any write operations made in the callback from the
   traverse.

 - the normal tdb_traverse() call allows for read or write operations
   in the callback, but gets the transaction lock, preventing
   transastions from starting inside the traverse

In addition we enforce the following rule that you may not start a
transaction within a traverse callback, although you can start a
traverse within a transaction

With these rules in place I believe all the deadlock possibilities are
removed, and we can now allow for searches to happen in parallel with
transactions
(This used to be commit 7dd31288a701d772e45b1960ac4ce4cc1be782ed)

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