Refactor JsonServlet's service startup to be easier to override
Refactor JsonServlet's service startup to be easier to override

We now do the service handle init before the xsrf init, allowing
the service handle init to possibly populate information used in
the xsrf init code path.

We also allow the service handle init to throw any checked exception,
turning into a servlet exception.  This makes it easier to write
service factory methods.

Signed-off-by: Shawn O. Pearce <f3ea74d906fa9fe97c1fef6bad9cb871485c7045@google.com>

CandyShop/gerrit.gwtjsonrpc,GerritCodeReview/gwtjsonrpc,evanchueng/gerrit.gwtjsonrpc,1yvT0s/gerrit.gwtjsonrpc,GerritCodeReview/gwtjsonrpc