If one thread was committing a transaction and another thread was
opening a new DB connection, deadlock could occur.  The cause of the
deadlock is that tpc_finish() acquires the storage and db locks in a
different order than DB.open().  As a result, if each starts at the
same time and gets one of the two locks it needs, the system will be
deadlocked.

The solution is to enforce a consistent locking order.  If a thread is
going to hold the DB lock and the storage lock, it MUST acquire the DB
lock first.  This patch implements that locking order for the
invalidation in tpc_finish().

The DB object gets methods called begin_invalidation() and
finish_invalidation() that acquire and release the DB lock
respectively.  Before the Connection calls tpc_finish() on the
storage, it calls begin_invalidation().  This guarantees that the DB
acquired before the storage lock.

When the invalidation phase is over, the Connection calls
end_invalidation() to release the DB lock.  This is an optimization.
It could wait until tpc_finish() returns, but we know that the DB will
not be used again for the rest of the tpc_finish() and tpc_finish()
could take a long time.

Specific changes:

DB.py
begin_invalidation(): Added.
finish_invalidation(): Added.
invalidate(): Remove locking.
invalidateMany(): Add comment about how it should be used.

Connection.py
tpc_finish(): Don't pass second argument to storage's tpc_finish()
    when committing a transaction with no data.  Add call to
    begin_invalidation() before calling storage's tpc_finish().
_invalidate_sub(): Remove empty, unnecessary method.
_invalidating_invalidating(): Add call to finish_invalidation() after
    last call to DB's invalidate().