Implement 'scan once' mode in online DQM input sources

[dmitrijus]

Completely ignores file "streaming" and uses only files found
at the initial scan. Useful for testing and development.

[cmsbuild]

A new Pull Request was created by @dmitrijus (Dmitrijus) for CMSSW_8_0_X.

It involves the following packages:

DQM/Integration
DQMServices/StreamerIO

@cmsbuild, @vanbesien, @deguio, @davidlange6 can you please review it and eventually sign? Thanks.
@barvic, @threus, @batinkov this is something you requested to watch as well.
@slava77, @Degano, @smuzaffar you are the release manager for this.

cms-bot commands are list here #13028

[deguio]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/11143/console

[cmsbuild]

+1
Tested at: 2508ff0
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-13250/11143/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-13250/11143/summary.html

[deguio]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_8_0_X IBs (tests are also fine). This pull request requires discussion in the ORP meeting before it's merged. @slava77, @davidlange6, @Degano, @smuzaffar