waitfornewblock waits until a new block is received, or the timeout expires, then
returns the current block height/hash.
waitforblock waits for a specific blockhash, or until the timeout expires, then
returns the current block height/hash. If the target blockhash is the current
tip, it will return immediately.
waitforblockheight waits until the tip has reached a certain height or higher,
then returns the current height and hash.
waitforblockheight is used to avoid polling in the rpc tests.
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