Dispatch-1055: Router fails to propagage credit on link route after drain cycle

[ChugR]

In this PR the "fix" is all in transfer.c.

• The router was not accounting for transfers that never arrive when drain consumes the credit without sending messages. The fix is to reset the internal credit counts when a drain cycle ends.
• The router calculates if drain changed more correctly.

The commit also adds a test that works when not in link route and fails in a link route.