optimize postgresql

optimize postgresql server by three things:

(1)change the max number of file descriptors to 10,000 per process
adding:
* soft nofile 10240
* hard nofile 20000

into /etc/security/limits.conf file
(2)change linux kernel shared memory to 400M.
adding: kernel.shmmax=400000000 into file /etc/sysctl.conf file and reboot the server

(3)change postgresql.conf file to modify the shared buffer space to 300M.

max connection still keep to 100. It should be enough.