Move synchronization loop into new package frontend

This is in preparation for other programs (besides cmd/safcm) using the
safcm library. To reduce code duplication useful functions will be
provided by the frontend package. Its use is optional. All core
functionality is provided by the regular safcm packages.

The logging setup was slightly modified. Log messages are now no longer
filtered by Sync.log() but by the new log function Loop.LogEventFunc (or
its implementation logEvent()). This is also the reason why one test was
removed from sync_sync_test.go which is no longer relevant.