MDEV-19964 S3 replication support

Added new configure options:
s3_slave_ignore_updates
"If the slave has shares same S3 storage as the master"

s3_replicate_alter_as_create_select
"When converting S3 table to local table, log all rows in binary log"

This allows on to configure slaves to have the S3 storage shared or
independent from the master.

Other thing:
Added new session variable '@@sql_if_exists' to force IF_EXIST to DDL's.