Deprecate pseudo mode

Previously, gerrit handler could not accept any events during
disconnected. So pseudo mode was added to be able to get events
in any connection status.

After implementation of GerritServer, handler can accept events
during disconnected without pseudo mode. So this patch removes it.
Add "No connection on bootup" option in server config

This patch adds a feature that connection to server is not established
on bootup.

Fix for JENKINS-24099

Task-Url: https://issues.jenkins-ci.org/browse/JENKINS-24099
Add transient modifier to pseudoMode
Unified wording to 'startup'
Add tests for GerritItemListener#onLoaded
Merge pull request #168 from rinrinne/boot-on-startup

Add "No connection on bootup" option