API correctness fixes for Transaction support

[darinf]

Changes:

• Fix signature of runTransaction's updateBlock parameter
• Add definition of NSErrorPointer since swift-corelibs-foundation does not provide it
• Make Transaction methods that return the Transaction to support chaining declare the result discardable so as to not generate warnings when the return value is ignored
• Switch to a fatalError (even in release builds) if trying to use a Transaction outside of an updateBlock
• Switch to non-mutating methods / const C++ methods so that a Transaction instance passed as a parameter to a function (updateBlock) can be used

[brianmichel]

Nice!