rpc: Compute number of confirmations of a block from block height
rpc: Compute number of confirmations of a block from block height

Currently this uses a CMerkleTx, but that makes no sense as we
have the CBlockIndex available. As noted by @jgarzik.

(cherry picked from commit 57153d4e1ac6e2cc9fc3f2fcd6d786e9813d824f)

reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin,reddcoin-project/reddcoin