Fix false-negatives for codec registration checker plugin

[LukaszKontowski]

In specific scenarios, codec registration checker plugin does not find missing codec registrations - i.e. when using sbt incremental compilation. Example scenario with explanation - steps to be performed on a project that uses ASH:

1. sbt compile on existing code (fresh compilation on all files) - compilation succeeds - OK
2. add new top level serializable class (in a separate file) and do not register it in serializer
3. sbt compile - compilation succeeds - NOT OK

If we would perform only steps 2. and 3. - sbt compile would end in an error (which is what we want) saying that there are missing codec registrations. So, this is an issue with incremental compilation. The reason of this issue is to fix such "false negatives". Compilation should never succeed for situations as described above - whether this is full compilation on all sources - or just an incremental compilation on one or more added files.

[LukaszKontowski]

This issue is related to the way CirceAkkaSerializer (CAS) works. For CAS the problem is that:

• codec registration checker plugin searches for org.virtuslab.ash.circe.Registration occurences in the compiled source
• CAS real registrations are based on the codecs collection - which must be defined in the CAS implementation in the client's code in a file that contains the top-level Serializer (let's call this file A.scala)
• In situations as described above - if we add a new file - B.scala with new Registrations - it would look OK to the plugin - because it has found Registrations of types that were to be checked
• However - as A.scala is not compiled and in effect it is not checked by the plugin - plugin will not find this missing Registration in A.scala

In fact - we would need to force compilation of the A.scala file in such situations to catch such errors.

[PawelLipski]

org.virtuslab.ash.circe.enable-missing-codecs-check config key can be removed if this is done, right?

[LukaszKontowski]

Yes, and connected part of logic - method checkSerializableTypesForMissingCodec