Merge pull request #7011
6531f17 Add mediantime field to getblock and getblockheader (Peter Todd)
7259769 Document new mediantime field in getblockchaininfo (Peter Todd)
c277a63 Clarify nLockTime-by-time comment in CheckFinalTx() (Peter Todd)
748321e Add mediantime field to getblockchaininfo RPC call (Peter Todd)