Fix bug when tracking multiple simultaneous spawned ping cycles

When more than 1 ping cycle is spawned simultaneously (for instance
REQUEST_PING + REQUEST_NODE_REGISTRATION_STATUS for the selected nodes),
we do not track a separate ping_start time for each cycle. When ping_begin()
is called, the information about the previous ping cycle is lost. Then when
ping_end() is called for the first of the two cycles, we set ping_start=0,
which is incorrectly used to see if the last cycle ran for more than
PING_TIMEOUT seconds (100s), thus incorrectly triggering the:

 error("Node ping apparently hung, many nodes may be DOWN or configured "
       "SlurmdTimeout should be increased");

Bug 3914