Requests information (generally previously announced via an INV) from a peer.
A GETDATA request is formatted as a vector of INV data:
|compact int||4 bytes||32 bytes||…||additional objects|
|vector size N elements of||element 1 type||element 1 hash||…||element N type and hash|
The type of the desired object. See INV for specific values
The hash identifier of the desired object.