logging: fix use-after-free

The pagespeed_connection needs a log to write to, but unfortunately
r->connection->log isn't guaranteed to still be around for the whole
lifetime of the pagespeed_connection.  Use the log from the server
context instead.

Fixes issue #522.
Merge pull request #535 from pagespeed/jefftk-close-connection-v3

logging: fix use-after-free