MINOR: log: pass the input end to the _lf_encode_bytes() byte encoders

MINOR: log: pass the input end to the _lf_encode_bytes() byte encoders

The byte encoding callbacks used by _lf_encode_bytes() only receive the
current byte, so they cannot look at the following ones. This will be
needed by the json encoder to detect multi-byte UTF-8 sequences. Let's
pass them <bytes_stop> as well as an end pointer, which is NULL for
NUL-terminated inputs.