iproto: call tx_check_msg before processing replication requests

In contrast to all other request handlers, replication request handlers
don't check the schema version. Strictly speaking, this is incorrect -
if it's specified it must always be checked. It was fine, because we
don't set the schema version in replication requests. However, in the
future we're planning to add more checks in tx_check_msg, which will
have to be performed for all requests, including replication. So let's
call tx_check_msg before processing replication requests - it shouldn't
hurt to check the schema version if available.

Needed for https://github.com/tarantool/tarantool-ee/issues/301

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