app-layer: configurable GetActiveTxId function

app-layer: configurable GetActiveTxId function

In preparation of a patchset that will allow for disabling the detect
module, this patch introduces a way to register a function for getting
the lowest active tx id. This is used by the app layer for cleaning up
transactions that already fully inspected, and by the flow timeout code
to determine if a flow is fully inspected and logged at timeout.

The registration function RegisterAppLayerGetActiveTxIdFunc allows for
registration of a custom function of type:
  uint64_t (*GetActiveTxIdFunc)(Flow *f, uint8_t flags);

If no function is called, AppLayerTransactionGetActiveDetectLog is used,
which implements the existing behaviour of considering both the
inspect_id's and the log_id.