MINOR: log: merge lf_encode_string() and lf_encode_chunk() logic

MINOR: log: merge lf_encode_string() and lf_encode_chunk() logic

lf_encode_string() and lf_encode_chunk() function are pretty similar. The
only difference is the stopping behavior, encode_chunk stops at a given
position while encode_string stops when encountering '\0'. Moreover,
both functions leverage tools.c encode helpers, but because of the
LOG_OPT_ESC option, they reimplement those helpers with added logic.

Instead of having to deal with code duplication which makes both functions
harder to maintain, let's define a _lf_encode_bytes() helper function
which satisfies lf_encode_string() and lf_encode_chunk() needs while
keeping the function as simple as possible.

_lf_encode_bytes() itself is made of multiple static inline helper
functions, in the attempt to keep checks outside of core loop for
better performance.