keepalive: set timestamp on core registration

This patch sets a timestamp on each lcore when it is registered
for keepalive. This causes the first values read by the monitor
to show time since the core was registered, instead of the delta
between 0 and the timestamp counter.

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Remy Horton <remy.horton@intel.com>