make ktap_lib_printf use general buffer(kp_percpu_buffer)

Reduce memory consuming.
use kp_tstring_newlstr_local in e.tostring()

Since e.tostring() use general percpu buffer, we cannot
use that buffer as static buffer, it will conflict with printf,
which use same general percpu buffer, so make e.tostring() use
dynamic allocated string, this will slow the function a little,
but it can be accepted, user should use print(e) if they want
to print string representation of event.
kill kp_tstring_assemble

there have no user for kp_tstring_assemble function at present.
remove incorret comments