Interface for high volume client submissions

[mtrippled]

async: This returns immediately, and is actually faster for the client. However, transaction submission errors that occur only once the transaction is applied will be unknown to the client. This should be used for performance testing.

Is it possible to asynchronously send an error message to the websocket connection that the client program can still handle later?

Originally posted by @justinr1234 in #4504 (comment)

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The best way to send concurrent transactions should be evaluated as transaction volumes increase and it becomes necessary for stability to either use the "async" or "wait" options for submitting.