Add response time statistics into sysguard module

Two new directives are introduced:
*) sysguard_rt_cache time, avail at main ctx only;
*) sysguard_rt rt=time period=time, avail at loc/srv/main ctx.

Build a ring to store at most "sysguard_rt_cache" long statistic data in
a shared memory. Fecthing results from the statistic database follows the
"sysguard_interval" principal.

According to the requirements, the response time functionality only
works together with the "load" functionality, using this feature alone will
take no effect.

Find more info in the test cases.

Test cases are included.

Signed-off-by: Paul Yang(InfoHunter) <paulyang.inf@gmail.com>