MEDIUM: log: optimizing tmp->type handling in sess_build_logline()

MEDIUM: log: optimizing tmp->type handling in sess_build_logline()

Instead of chaining 2 switchcases and performing encoding checks for all
nodes let's actually split the logic in 2: first handle simple node types
(text/separator), and then handle dynamic node types (tag, expr). Encoding
options are only evaluated for dynamic node types.

Also, last_isspace is always set to 0 after next_fmt label, since next_fmt
label is only used for dynamic nodes, thus != LOG_FMT_SEPARATOR.

Since LF_NODE_WITH_OPT() macro (which was introduced recently) is now
unused, let's get rid of it.

No functional change should be expected.

(Use diff -w to check patch changes since reindentation makes the patch
look heavy, but in fact it remains fairly small)