Fix Gerrit plugins under Tomcat by avoiding Guice static filter

The static GuiceFilter breaks when a plugin loads and configures
its own web environment. Follow the pattern used by JettyServer
to initialize and load GuiceFilter in a non-static way, allowing
Gerrit to always have its own filter stack during any request.

Bug: issue 1966
Change-Id: I864f3badc4ec90154a276d35e6a5a3fa75058933