pgsql: Provide interim statistics while in mid-checkpoint.

Provide interim statistics while in mid-checkpoint.
Re-implements similar functionality in 9.1 and previously which
was removed during split of checkpointer and bgwriter.

Requested/spotted by Magnus Hagander

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/1ec6a2bbc966a05acf10a6753d8ec7db83f43f4c

Modified Files
--------------
src/backend/postmaster/checkpointer.c |    5 +++++
1 files changed, 5 insertions(+), 0 deletions(-)