trace: ensure buffers are initialized when tracepoint is enabled

Tracepoints can be enbaled not only via enable_tracepoint(std::string)
but via tracepoint_base::enable() also. This change also makes the
initialization thread-safe as it may be called from aribtrary thread.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>