detect: make detect engine types explicit

detect: make detect engine types explicit

There are 3 types of detect engine objects:
    1. normal
       The normal detection engine if no multi-tenancy is in use

    2. tenant
       A per tenant detection engine

    3. stub
       A stub (or minimal as it was called before) detect engine
       that is needed to have something in place when there are
       only tenants.

       A stub is also used in case of 'delayed detect', where we
       need a minimal detect engine to start up which is replaced
       by a full (normal type) detect engine after startup.

This patch adds a new field 'type' to the DetectEngineCtx object
to distinguish between the types. This replaces the boolean 'minimal'.