THRASHING TEST.  Fill the table to exactly full, then repeat the
block  INSERT, DELETE, DELETE, INSERT  until n/2 operations are done.

n =    1024
  A: shrink below 1/2  table=     256 ops=      512 cost=       65920 cost/op=     128.75  expansions=     128 contractions=128
  B: shrink below 1/4  table=     256 ops=      512 cost=         768 cost/op=       1.50  expansions=       1 contractions=0
n =    4096
  A: shrink below 1/2  table=    1024 ops=     2048 cost=     1050112 cost/op=     512.75  expansions=     512 contractions=512
  B: shrink below 1/4  table=    1024 ops=     2048 cost=        3072 cost/op=       1.50  expansions=       1 contractions=0
n =   16384
  A: shrink below 1/2  table=    4096 ops=     8192 cost=    16783360 cost/op=    2048.75  expansions=    2048 contractions=2048
  B: shrink below 1/4  table=    4096 ops=     8192 cost=       12288 cost/op=       1.50  expansions=       1 contractions=0
n =   65536
  A: shrink below 1/2  table=   16384 ops=    32768 cost=   268460032 cost/op=    8192.75  expansions=    8192 contractions=8192
  B: shrink below 1/4  table=   16384 ops=    32768 cost=       49152 cost/op=       1.50  expansions=       1 contractions=0
n =  262144
  A: shrink below 1/2  table=   65536 ops=   131072 cost=  4295065600 cost/op=   32768.75  expansions=   32768 contractions=32768
  B: shrink below 1/4  table=   65536 ops=   131072 cost=      196608 cost/op=       1.50  expansions=       1 contractions=0

Potential and worst case on a long mixed run (400000 inserts, then 400000 deletes):
  shrink below 1/2   cost=  1848555  cost/op=2.3107  worst single op=  262145  lowest potential=0  final num=0 size=1
  shrink below 1/4   cost=  1586412  cost/op=1.9830  worst single op=  262145  lowest potential=0  final num=0 size=2
