Change the implementation of hash join to attempt to avoid unnecessary

Change the implementation of hash join to attempt to avoid unnecessary
work if either of the join relations are empty. The logic is:

(1) if the inner relation's startup cost is less than the outer
    relation's startup cost and this is not an outer join, read
    a single tuple from the inner relation via ExecHash()
      - if NULL, we're done

(2) read a single tuple from the outer relation
      - if NULL, we're done

(3) build the hash table on the inner relation
      - if hash table is empty and this is not an outer join,
        we're done

(4) otherwise, do hash join as usual

The implementation uses the new MultiExecProcNode API, per a
suggestion from Tom: invoking ExecHash() now produces the first
tuple from the Hash node's child node, whereas MultiExecHash()
builds the hash table.

I had to put in a bit of a kludge to get the row count returned
for EXPLAIN ANALYZE to be correct: since ExecHash() is invoked to
return a tuple, and then MultiExecHash() is invoked, we would
return one too many tuples to EXPLAIN ANALYZE. I hacked around
this by just manually detecting this situation and subtracting 1
from the EXPLAIN ANALYZE row count.