Check if message is mined

In order to check if a particular message cid is mined you can check if it's in the blockchain store:

 const httpConnector = new HttpJsonRpcConnector({ url: __LOTUS_RPC_ENDPOINT__, token: __LOTUS_AUTH_TOKEN__ });
 const con = new LotusClient(httpConnector);
 const isMined = await con.chain.hasObj(__MSG_CID__);