Implement read-only transaction manager modes for R3.0 migration (#1241)

This involves:
- Altering both transaction managers to check for a read-only mode at
the start of standard write actions (e.g. delete, put).
- Altering both raw layers (entity manager, ofy) to throw exceptions on
write actions as well
- Implementing bypass routes for reading / setting / removing the schedule itself
so that we don't get "stuck"

config/presubmits.py

@@ -215,6 +215,7 @@ PRESUBMITS = {
          "RdapDomainSearchAction.java",
          "RdapNameserverSearchAction.java",
          "RdapSearchActionBase.java",
+         "ReadOnlyCheckingEntityManager.java",
          "RegistryQuery",
          },
     ):