Change 21207

Paul Allen
Multi client support in Pipeline.

If two or more `p4sync` operations are called in one Pipeline script, they MUST have different client names.  During a build, multiple entries are added to the build history; on subsequent builds they are used in the three following situations:
 - Polling: largest change number across all the checkouts is used.
 - Change Summary: last change with the same client name is used.
 - Environment: last change recorded (regardless of client).

Added two test cases to cover basic multi sync build/poll situations.

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