REST transactions: transaction handling and request format changes.

o Delay resuming transactions until actual execution time, instead
  of resuming immediately after retrieving from registry.
o Wrap statements in a top-level object instead of a simple list.
o Note that there are serious limitations in the parsing mechanism,
  but fixing those limitations will be delayed until a later story
  when we add the ability to specify per-transaction timeouts.
Merge pull request #705 from apcj/2.0-rest-transactions

REST transactions: transaction handling and request format changes.