client rejected message sent by peer

After firstly synced, when witness_node is in normal running mode, I noticed lots of messages in `p2p.log` (with log level `warn`):

> p2p:message read_loop process_ordinary_mes ] client rejected message sent by peer xxx.xxx.xxx.xxx {"code":10,"name":"assert_exception","message":"Assert Exception","stack":[{"context":{"level":"error","file":"db_block.cpp","line":579,"method":"_apply_transaction","hostname":"","thread_name":"th_a","timestamp":"x"},"format":"(skip & skip_transaction_dupe_check) || trx_idx.indices().get().find(trx_id) == trx_idx.indices().get().end():

This occurs when p2p failed to push a transaction to object database.

Possible optimization:
* p2p layer don't push duplicate transactions / blocks to chain logic layer. I think there is already a mechanism about this, but perhaps possible to improve, btw #517 is related.
* Reduce level of this logging message if it's expected to occur a lot

Thoughts?