Add pgstathashindex() to get hash index table statistics.

Hi All,

I have introduced a new function 'pgstathashindex()' inside pgstatuple
extension to view the statistics related to hash index table. I could
have used 'pgstattuple()' function to view hash index stats instead of
adding this new function but there are certain limitations when using
pgstattuple() for hash indexes. Firstly, it doesn't work if a hash
index contains zero or new pages which is very common in case of hash
indexes. Secondly, it doesn't provide information about different
types of pages in hash index and its count. Considering these points,
I have thought of introducing this function. Attached is the patch for
the same. Please have a look and let me your feedback. I would also
like to mention that this idea basically came from my colleague Kuntal
Ghosh and i  implemented it. I have also created a commit-fest entry
for this submission. Thanks.

With Regards,
Ashutosh Sharma
EnterpriseDB:http://www.enterprisedb.com