YOLO! Major Simplification to updatePayment*, Unified Reorg

* No longer store lastBlockHash in CouchDB, close issue #45.
* lastBlockJob's listSinceBlock is missing double-spent transactions so it was failing to update.
  Meanwhile its reorg handling was redundant to watchpaymentjob and walletnotify.
  Removing reorg handling from lastBlockJob broke reorg_history entirely.
  It turns out reorg handling in other codepaths was broken by a few other errors.
  * validateTransactionBlock() isReorg was always false, logic bug.
  * updatePaymentWithTransaction()'s reorg handling could not work because blockIsValid is
    irrelevant to whether or not you should handle reorg/double-spend.
  * processReorged* functions were encouraging different windy entry points into reorg handling,
    in addition to being inflexible and difficult to trace.

It was simpler to rewrite updatePaymentWithTransaction() to unify all entry points of updatePayment* and
to always handle reorg/double-spent in the same code-path.

It seems to work.  All tests pass.

TODO: There is a new noisy "Document update conflict" that we need to detect and hide.