!per #15230 rename processorId => persistentId

* This is NOT binary compatible, we're in an *experimental* module.
* disabled binary compat checks for package akka.persistence
* Source compatibility is retained, but users should migrate do the new
  method name ASAP.
* Plugin APIs were migrated in a way that allows the old plugins to
  compile agains 2.3.4 without having to change anything. Hopefuly this
  will help authors migrate to 2.3.4 sooner. This is only source level compatible, not binary compatible.
* added deprecation warnings on all processorId methods and provided bridges where possible
* for users, the migration should be painless, they can still override
  the old method, and it'll work. But we encourage them to move to
  persistenceId; All delegation code will have to be removed afterwards ofc.

Conflicts:
	akka-persistence/src/main/scala/akka/persistence/Channel.scala
	akka-persistence/src/main/scala/akka/persistence/JournalProtocol.scala
	akka-persistence/src/main/scala/akka/persistence/Persistent.scala
	akka-persistence/src/main/scala/akka/persistence/PersistentChannel.scala
	akka-persistence/src/main/scala/akka/persistence/Processor.scala
	akka-persistence/src/main/scala/akka/persistence/Snapshot.scala
	akka-persistence/src/main/scala/akka/persistence/journal/AsyncWriteProxy.scala
	akka-persistence/src/main/scala/akka/persistence/journal/inmem/InmemJournal.scala
	akka-persistence/src/main/scala/akka/persistence/journal/leveldb/LeveldbKey.scala
	akka-persistence/src/main/scala/akka/persistence/snapshot/SnapshotStore.scala
	akka-persistence/src/test/scala/akka/persistence/serialization/SerializerSpec.scala
	project/AkkaBuild.scala
Merge pull request #15434 from ktoso/port-persistenceId-ktoso

!per #15230 rename processorId => persistentId (for validation)