Various improvements for multithreading

[wmtan]

No description provided.

[wmtan]

This pull request is a collection of several changes to prepare for the multithreaded framework.
Here are the changes:

1. Functions ParameterSetEntry::pset(), VParameterSetEntry::vpSet(), ThreadSafeRegistry::data(), and ThreadSafeRegistry::extra() came in both const and non-const versions, making it difficult to tell when the underlying data was modified. The non-const versions of these functions were renamed by appending a "ForUpdate" to each function name. This is the only change that affected the DQM, HLT, and Visualization packages, each of which called one or more of these functions.
2. The value_ptr template in FWCore/Utilities was improved by adding a constructor and an assignment operator from a std::unique_ptr.
3. A new template, atomic_value_ptr, was added to FWCore/Utilities. This is similar to a value_ptr, but the underlying pointer used is a std::atomic<T*>, rather than a T*, which allows atomic operations. A unit test was added for the new template.
4. An underscore was appended to the name of each data member of VParameterSetEntry, making the naming consistent with ParameterSetEntry and other framework classes.
5. The data members ParameterSetEntry::theID_, VParameterSetEntry::theIDs_, and ParameterSet::id_ were made non-mutable.
6. The data members ParameterSetEntry::thePSet_ and VParameterSetEntry:theVPSet_ were changed from a value_ptr to an atomic_value_ptr so they could be updated in a thread safe manner.
7. Some functions related to parameter sets had their signatures modified to take arguments by reference or const reference rather than by pointer, where this change was appropriate.
8. BranchKey was changed from a struct to a class to avoid public data members.
9. Mutables in History and *Aux were made non-mutable.
10. Some C++11 improvements in affected code, namely range for loops and the use of nullptr.

[nclopezo]

Hi

I took CMSSW_7_0_X_2013-07-30-1400, pulled these changes, ran the unit tests and the RelVals. All tests passed.

[cmsbuild]

The following categories have been signed by chrjones (a.k.a. @Dr15Jones on GitHub): Core, Visualization

@cms-git-visualization, @cms-git-core, @cms-git-geometry

[ktf]

@rovere @deguio @Martin-Grunewald can you please check this is fine with you and sign?

[cmsbuild]

The following categories have been signed by @deguio: DQM

@cms-git-dqm

[cmsbuild]

The following categories have been signed by gruenew (a.k.a. @Martin-Grunewald on GitHub): HLT

@cms-git-hlt, @cms-git-fastsim

[cmsbuild]

The following categories have been signed by gruenew (a.k.a. @Martin-Grunewald on GitHub): HLT

@cms-git-hlt, @cms-git-fastsim

[cmsbuild]

The following categories have been signed by gruenew (a.k.a. @Martin-Grunewald on GitHub): HLT

@cms-git-hlt, @cms-git-fastsim

[cmsbuild]

The following categories have been signed by gruenew (a.k.a. @Martin-Grunewald on GitHub): HLT

@cms-git-hlt, @cms-git-fastsim