[swift-4.0-branch][overlay] Rename scheduleOneShot and scheduleRepeating

[moiseev]

• Explanation: This change deprecates several methods of the DispatchSourceTimer struct, replacing them with methods that have more meaningful names. It also introduces the enumeration case DispatchTimeInterval.never, which can be used to specify that a timer should never fire or never repeat.
• Scope of Issue: This is not a source-breaking change. The only effect on existing code is that uses of the deprecated methods will be flagged at compile time.
• Risk: Since the existing methods remain and their code is almost unmodified, this is a very low-risk change.
• Reviewed By: Daniel Steffen
• Testing: There are tests in the Dispatch test suite that cover this code change.
• Directions for QA: N/A
• Radar: rdar://problem/32991313

[moiseev]

Cherrypicked from #10598

[moiseev]

@swift-ci Please test