Avoid linkDef files for root6

[wmtan]

Although the format of LinkDef.h files is the same in ROOT5 and ROOT6, the effects are not necessarily the same.
Because of this, some needed dictionaries were not loaded. and many framework unit tests failed against ROOT6.
LinkDef.h files were used in the framework for historical reasons only. The easiest way to fix the test errors is to use classes_def.xml and classes.h consistently throughout the framework for generating ROOT6 dictionaries.
Some minor refactoring of code in IOPool/TFileAdaptor was needed to do this.
While we could in principle do the same for ROOT 5, this is non-trivial in ROOT5 because two of the classes in question inherit from TObject. Because of the complexities of CINT, Cintex, and Reflex, additional work would be needed for ROOT5.
This pull request fixes many framework unit test errors against ROOT6.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_0_ROOT6_X.

Avoid linkDef files for root6

It involves the following packages:

FWCore/FWLite
IOPool/TFileAdaptor

@cmsbuild, @Dr15Jones, @ktf, @nclopezo can you please review it and eventually sign? Thanks.
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[Dr15Jones]

+1
Just to correct a statement in Bill's pull request. We can't use this in ROOT5 since the LinkDef.h were used explicitly to avoid having to use Cintex. As Bill pointed out in an offline conversation, ROOT6 doesn't have Cintex so that complication goes away.

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next IBs unless changes or unless it breaks tests.