logger: s/\<log\>/log_/

logger: s/\<log\>/log_/

Currently the MSVC build is broken because of conflicting definitions of
'log' function.  I didn't investigate thoroughly, but I suspect the
it is conflicting standard math.h's log.

log_ is admittedly not a great name, but it is better than a broken build.
A better one can be used in a follow-on build.