Replace mark-and-delete at next merge algorithm in DQMStore (76x)

[cmsbuild]

Additionally, the DQMStore will now acquire locks for writing files.

This PR also add a new file saver for HLT->PB (DQMFileSaverPB).
Which should replace DQMFileSaver in the near future.

(Discussion: #11086)
Automatically ported from CMSSW_7_6_X #11251 (original by @dmitrijus).

[cmsbuild]

A new Pull Request was created by @cmsbuild for CMSSW_8_0_X.

Replace mark-and-delete at next merge algorithm in DQMStore (76x)

It involves the following packages:

DQMServices/Components
DQMServices/Core
DQMServices/FileIO

The following packages do not have a category, yet:

DQMServices/FileIO

@cmsbuild, @danduggan, @vanbesien, @deguio can you please review it and eventually sign? Thanks.
@barvic this is something you requested to watch as well.
You can sign-off by replying to this message having '+1' in the first line of your reply.
You can reject by replying to this message having '-1' in the first line of your reply.
If you are a L2 or a release manager you can ask for tests by saying 'please test' in the first line of a comment.
@Degano you are the release manager for this.
You can merge this pull request by typing 'merge' in the first line of your comment.