Bushy Plans fixed

I have fixed bushy plans, enabled with the postgres -b option.  Here is
proof from OPTIMIZER_DEBUG:
levels left: 7(9 8 7 6 ): size=1 width=16        path list:        Nestloop size=1 cost=0.000000
MergeJoinsize=1 cost=0.000000                           clauses=(x5.y = x6.y)                           sortouter=1
sortinner=1                       SeqScan(6) size=0 cost=0.000000                        SeqScan(7) size=0
cost=0.000000               MergeJoin size=1 cost=0.000000                           clauses=(x7.y = x8.y)
            sortouter=1 sortinner=1                        SeqScan(8) size=0 cost=0.000000
SeqScan(9)size=0 cost=0.000000 
 

The regression tests pass with bushy plans enabled.  I am not sure if
the executor is actually using a bushy plan, though.

The old bushy code was poor.  It tried to do bushy plans by modifying
the joininfo nodes.  I removed all that code, and just do the join
search in make_rels_by_clause_joins().  This is much more logical, and
does not require the joininfo setup/cleanup that the old code attempted.

Frankly, I would like to enable bushy plans and right-handed plans by
default.  The optimizer is now fast enough that a 9-table join is almost
instantaneous, even with bush plans.  People are not sophisticated
enough to know when to enables these options themselves. I am not sure I
am either.  I would like to enables both, fix whatever breaks, and
programmatically enable the options when they make sense.

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