Port back from ROOT6 IB for commonality (Core)

[wmtan]

This PR ports back simple changes made in CMSSW_7_4_ROOT6_X in the Core L2 category that are desirable or harmless for CMSSW_7_4_X, in order to increase code commonality. All affected files will be identical in the two releases after this PR is merged. Changes were successfully tested with the short relval matrix and unit test.
The changes are:

1. An extra include needed for ROOT6.
2. Code featured out by conditional compilation was changed to a comments.
3. A "long long" was replaced by a more descriptive typedef.
4. Macro exit codes used only in unit tests changed from 1 to 0. 0 is correct. Tests still pass.
5. an unnecessary forward declaration was removed.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_4_X.

Port back from ROOT6 IB for commonality (Core)

It involves the following packages:

DataFormats/FWLite
FWCore/FWLite
IOPool/Output

@cmsbuild, @Dr15Jones, @ktf, @nclopezo can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @wddgit 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.
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@Degano you are the release manager for this.
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