Added preallocate signal for Services

[Dr15Jones]

Services can now listen for the preallocate Service in order to determine the maximum number of concurrent Streams, Lumis and Runs the system will use for the job.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_0_X.

Added preallocate signal for Services

It involves the following packages:

FWCore/ServiceRegistry
FWCore/Framework

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

+1

This adds a new signal not used by any Services yet so there is no possibility it could break anything. Just to be paranoid, I did run all the framework unit tests and everything passed.

In my opinion, this is safe to merge without any further testing.

[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.