Bug#25828806: GR ALLOWS OPERATIONS ON NON-COMPLAINT TABLES THROUGH MYSQLBINLOG

PROBLEM STATEMENT:
==================
We have few restrictions in GR such as:
1. operations on a table without PK is not allowed when running group
replication.
2. only operations on table with innodb storage engine is supported with GR
3. If a table has foreign key with a CASCADE clause, then operations on
such table is not allowed (in multi-primary mode)

But GR allows all these operations through asynchrnous replication and
mysqlbinlog utility when the operations are based on RBR events.

        GR                Async
M2 <------------> M1 <--------------M3
                slave              master

SOLUTION:
=========
A new hook (applier_log_event) is created to validate if a binary log
event should be applied or not. GR plugin on initialization subscribes
to this hook and using its validation logic ensures that all its requirements
are fulfilled.

Rows_log_event::do_apply_event() is now calling all the observer functions,
subscribed to this hook (applier_log_event), after opening and locking the
tables to be updated. This allows GR to verify if the tables are in compliance
with its requisites.