dpdkevents: detect primary DPDK application (re)start

dpdkevents: detect primary DPDK application (re)start

Current implementation attempts to open KA SHM only once during init callback.
If at the time primary DPDK application is not running shm_open will fail, which
will result in dpdkevents plugin being unloaded. Move KA SHM open logic to read
callback to allow plugin to attach to primary DPDK application later without a
need to restart collectd. Moreover, it will allow to determine if primary DPDK
application was killed and plugin should stop dispatching KA statistics.

Additionally modify dpdk_event_keep_alive_shm_open function to detect restart of
primary DPDK application. When primary DPDK application (with KA support) is
restarted it unlinks existing shared memory object and creates a new one. It's
necessary for the plugin to create a new SHM mapping in order to prevent reading
statistics from stale SHM block.

Change-Id: I7fc0047ba117db771ada58a40cbe0b2bddd00112
Signed-off-by: Przemyslaw Szczerbik <przemyslawx.szczerbik@intel.com>