Ensure HttpLog can always get the user identity

Preload the user before the request starts, to ensure we have it when
the request is over for the log file.  If we don't preload before
dispatching, a database connection may not be available at the end
of the request if nobody else has asked for the CurrentUser object.

Change-Id: Ic82a4b9d15705a20b0425b292fa14b78b934d97f
Signed-off-by: Shawn O. Pearce <sop@google.com>