The check for streaming replication logging format in
THD::decide_logging_format() did the check also for DDLs running
in TOI mode. This caused DROP DATABASE to fail if streaming
replication was enabled.

Added check for THD wsrep execution mode and perform the check
only if the THD is in local processing mode (i.e. not TOI).

Added galera_sr_create_drop test to verify that CREATE/DROP
statements pass even if streaming replication is on.