Sequentially built matrix project can cause backup plugin to never start backup
procedure and block hudson from building forever.

This is happened when backup procedure is started between start of project build
and start of configuration build. This is very likely to take place if matrix
project codebase is big (so checkout and build takes a long time) and sequential
compilation is enabled.

Backup plugin sets shutdown mode so configuration are not started. After that
plugin waits for running projects to be finished. But 'main' project waits for
configurations before finish itself. So we have a deadlock.