In galera cluster, when myisam replication is enabled
(wsrep_replicate_myisam=ON), DML statements are replicated
in open_tables(). However, in case of prepared statements,
for an INSERT, open_tables() gets invoked twice. Once for
COM_STMT_PREPARE (to validate and prepare INSERT) and later
for COM_STMT_EXECUTE. As a result, the command gets replicated
twice. Same happens for REPLACE, UPDATE and DELETE commands.
Fixed by adding a check to not replicate during 'prepare'
phase. Also changed the order of conditions to make it more
efficient. Lastly, in order to support wsrep_dirty_reads, made
changes to allow COM_STMT_XXX commands to continue past initial
check even when wsrep is not ready.