Debugger: Be less eager on breakpoint synchronization

Triggering synchronization too early leads to unnecessary pending
breakpoints and complicates debugger startup.

Change-Id: Ic1c0064f29eaa325a110baf9b22ec6bf66a00a2d
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>