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.