Release connection into pool with "Discard All"(new)

Hi,francisco
This is a new patch to replace #54,please give a look.

I had tested it for PG8.4 and PG9.2,all cases passed.And the performance also look like before.

run test cases without  this pacth:
tests runs:645
Time:52.936

run test cases with  this pacth:
tests runs:647
Time:52.897


Chen Huajun