Implement support for setting a single schema with all of its contents
to be 'read only' using the SQL statement:

  ALTER SCHEMA <schema name> READ ONLY [=] {0,1,DEFAULT}

Setting a schema to read only means that DDL modifying the schema or
any schema qualified entities will fail. It also means that write
operations on user data stored in one of the tables in the schema will
fail.

The read only option is ignored by bootstrap threads, upgrade code,
and slave replication threads.

In addition to the above functionality, some unrelated issues found
during testing have been fixed:

* While updating the validity of referencing views, we acquire
  referencing views and their schema based on object ids, and use
  their names to acquire MDL, both for the view and for the schema.
  However, we did this using the lettercase stored in the DD,
  whereas this should be lowercased when lower_case_table_names == 2.

* If an error was raised when the event scheduler was trying to
  schedule a new event, the scheduler would stop without logging
  an error message stating what the error was.

Change-Id: I7722bfecf6c97e3253ce795851e5ce1d5aa61f15