MINOR: log: postpone conversion for sample expressions in sess_build_logline()

MINOR: log: postpone conversion for sample expressions in sess_build_logline()

In sess_build_logline(), for sample expression nodes, instead of directly
calling sample_fetch_as_type(... SMP_T_STR), let's first process the
sample using sample_process(), and then proceed with the conversion to
str if required.

Doing so will allow us to implement type casting and preserving logic.