vlib: copy trace_handle in vlib_buffer_copy/clone() functions

vlib: copy trace_handle in vlib_buffer_copy/clone() functions

Since vlib_buffer_copy() and vlib_buffer_clone() both preserve
VLIB_BUFFER_IS_TRACED bit in flags field, it should also copy
trace_handle which would add minimal overhead. Thus, callers of
these functions do not have to call vlib_buffer_copy_trace_flags()
to copy trace_handle.

Type: refactor
Ticket: VPP-1745

Signed-off-by: John Lo <loj@cisco.com>
Change-Id: Iff6a3f81660dd62b36a2966033eb380305340310
(cherry picked from commit 6631780422787166550b9871c46a94c160416324)
Signed-off-by: John Lo <loj@cisco.com>