Duration is currently used only for testing, so that the Ping rpc
can simulate a delay, to create many simultaneous connections. Units
are microseconds.
PingResponse is used to indicate concurrency, how many Ping rpcs
are executing upon entry and upon exit (after the delay).
This rpc is used for testing only.
RawTransaction contains the complete transaction data. It also optionally includes
the block height in which the transaction was included, or, when returned
by GetMempoolStream(), the latest block height.
A SendResponse encodes an error code and a string. It is currently used
only by SendTransaction(). If error code is zero, the operation was
successful; if non-zero, it and the message specify the failure.
A TxFilter contains the information needed to identify a particular
transaction: either a block and an index, or a direct transaction hash.
Currently, only specification by hash is supported.