adds wrapper to hard copy write events for opencl copies

[SteveBronder]

Submission Checklist

• Run unit tests: ./runTests.py src/test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary

For stan-dev/math#2905 I need to make a hard copy of the event vector for OpenCL before making a copy to go from a tbb concurrent vector to a standard vector.

Intended Effect

In the downstream PR above the stan tests should now pass

How to Verify

Side Effects

Documentation

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)