lib: Convert RUSAGE_SELF to RUSAGE_THREAD where we can

When using getrusage, we have multiple choices about what
to call for data gathering about this particular thread of execution.

RUSAGE_SELF -> This means gather all cpu run time for all pthreads associated
with this process.

RUSAGE_THREAD -> This means gather all cpu run time for this particular
pthread.

Clearly with data gathering for slow thread as well as `show thread cpu`
it would be preferable to gather only data about the current running
pthread.  This probably was the original behavior of using RUSAGE_SELF
when we didn't have multiple pthreads.  So it didn't matter so much.

Prior to this change, 10 iterations of 1 million routes install/remove
from zebra would give us this cpu time for the dataplane pthread:

Showing statistics for pthread Zebra dplane thread
--------------------------------------------------
                      CPU (user+system): Real (wall-clock):
Active   Runtime(ms)   Invoked Avg uSec Max uSecs Avg uSec Max uSecs  Type  Thread
    0     280902.149    326541      860   2609982      550   2468910    E  dplane_thread_loop

After this change we are seeing this:

Showing statistics for pthread Zebra dplane thread
--------------------------------------------------
                      CPU (user+system): Real (wall-clock):
Active   Runtime(ms)   Invoked Avg uSec Max uSecs Avg uSec Max uSecs  Type  Thread
    0      58045.560    334944      173    277226      539   2502268    E  dplane_thread_loop

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>

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