vppinfra: improve clocks_per_second convergence

vppinfra: improve clocks_per_second convergence

Apply exponential smoothing to the clock rate update calculation in
clib_time_verify_frequency(), with a half-life of 1 minute and a
sampling frequency of 16 seconds. Within 5 minutes or so, the
calculation converges

With each rate recalculation: reset total_cpu_time based on the kernel
timebase delta since vpp started, and the new clock rate

Improve the "show clock [verbose]" debug CLI command.

BFD echo + echo fail tests marked off until the BFD code can be
reworked a bit.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I24e88a78819b12867736c875067b386ef6115c5c