Increase number of hash join buckets for underestimate.

 Increase number of hash join buckets for underestimate.
 
 If we expect batching at the very beginning, we size nbuckets for
 "full work_mem" (see how many tuples we can get into work_mem,
 while not breaking NTUP_PER_BUCKET threshold).
 
 If we expect to be fine without batching, we start with the 'right'
 nbuckets and track the optimal nbuckets as we go (without actually
 resizing the hash table). Once we hit work_mem (considering the
 optimal nbuckets value), we keep the value.
 
 At the end of the first batch, we check whether (nbuckets !=
 nbuckets_optimal) and resize the hash table if needed. Also, we
 keep this value for all batches (it's OK because it assumes full
 work_mem, and it makes the batchno evaluation trivial). So the
 resize happens only once.
 
 There could be cases where it would improve performance to allow
 the NTUP_PER_BUCKET threshold to be exceeded to keep everything in
 one batch rather than spilling to a second batch, but attempts to
 generate such a case have so far been unsuccessful; that issue may
 be addressed with a follow-on patch after further investigation.
 
 Tomas Vondra with minor format and comment cleanup by me
 Reviewed by Robert Haas, Heikki Linnakangas, and Kevin Grittner