add waitfornewblock/waitforblock/waitforblockheight rpcs and use them for tests

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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