OPTIM: log: use thread local lf_buildctx to stop pushing it on the stack

OPTIM: log: use thread local lf_buildctx to stop pushing it on the stack

Following previous commit's logic, let's move lf_buildctx ctx away from
sess_build_logline() to stop abusing from the stack to push large
structure each time sess_build_logline() is called. Also, don't memset
the structure for each invokation, but only reset members explicitly when
required.

For that we now declare one static lf_buildctx per thread (using
THREAD_LOCAL) and make sess_build_logline() refer to it using a pointer.