Relax the locking requirements on BTree cursors. Any number of read and

Relax the locking requirements on BTree cursors.  Any number of read and
write cursors can be open at the same time now, but a write cannot occur
as long as one or more read cursors are open.

Before this change, one or more read cursors could be open on a table,
or a single write cursor, but not both.  Both policies have the same
desirable effect: they prevent writes to a table while a sequential scan
of that table is underway.  But the new policy is a little less restrictive.
Both policies prevent an UPDATE from occurring inside a SELECT (which is
what we want) but the new policy allows a SELECT to occur inside an
UPDATE. (CVS 739)

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