Add mediantime to getblockchaininfo

[petertodd]

Useful now that BIP113 is enforced for transactions entering the mempool. Was previously only (indirectly) available by calling getblocktemplate, a relatively expensive RPC call.

Also, clarified a comment in CheckFinalTx()

[gmaxwell]

Do we also want to add the value to the verbose getblock output?

[petertodd]

@gmaxwell Good idea; done.

[gmaxwell]

Tested ACK.. but a question: are people going to get confused when they see median time in getblock output, but the nlock time thats enforced there is the mediantimepast?

[paveljanik]

ACK

[maflcko]

utACK

[petertodd]

@gmaxwell They probably will be. OTOH, they can always check the mediantime for the previous block. Also, this way avoids the special case for the genesis block.

edit: Also, if it's the mediantimepast, how do I get the mediantime for the current block? I'd have to special-case calling getblockchaininfo, and that might refer to a newer block than I was expecting.

[gmaxwell]

Fair enough.

[gmaxwell]

ACK.