IMPALA-4689: Fix computation of last active time
The last active time in impala-server.cc#L1806 is in milliseconds, but
the TimestampValue c'tor expects seconds. This change also renames some
variables to make their meaning more explicit, aiming to prevent similar
bugs in the future.
This change also fixes a bug that occurred when during startup of the
local minicluster the operating system PIDs would wrap around. This way
the first impalad would not be the one with the smallest PID and
ImpalaCluster.get_first_impalad() would return the wrong one.
I ran git-clang-format on the change.
Reviewed-by: Thomas Tauber-Marshall <email@example.com>
Reviewed-by: Marcel Kornacker <firstname.lastname@example.org>
Reviewed-by: Tim Armstrong <email@example.com>
Tested-by: Internal Jenkins