Allow console logger to log to alternate FILE

This allows the console logger to log somewhere other than stderr. This
may be controlled by a (currently undocumented) environment variable
`LCB_LOGFILE`.

Change-Id: I916321bbe8bb137da8c521851fa7d7b6b6b72f55
Reviewed-on: http://review.couchbase.org/42167
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>