commit 6294f3283a5b6919795621dc067ec80c0cd2a334
Author: John Newbery <john@johnnewbery.com>
Date:   Fri Feb 10 11:04:13 2017 -0500

    gettxoutproof() should return consistent result

    We can call gettxoutproof() with a list of transactions. Currently, if
    the first transaction is unspent (and all other transactions are in the
    same block), then the call will succeed. If the first transaction has
    been spent, then the call will fail. The means that the following two
    calls will return different results:

    gettxoutproof(unspent_tx1, spent_tx1)
    gettxoutproof(spent_tx1, unspent_tx1)

    This commit makes behaviour independent of transaction ordering by looping
    through all transactions provided and trying to find which block they're in.

    This commit also increases the test coverage and tests more failure
    cases for gettxoutproof()