Multi-document "Pando" MongoDriver format #71
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Make sure we're starting from a clean slate.
Turns out all the pain of dealing with Document vs BsonDocument was self-inflicted. We can just deal with BsonDocument everywhere and benefit from increased type safety.
Transactions don't work the way I assumed, so I've decided to make a wrapper class, TransactionalCollection, that works the way I want. It maintains a ThreadLocal<ClientSession> and injects that into every operation, ensuring that we won't accidentally do a one-off operation that punches through the transaction. It's also flexible about transaction semantics. Code that needs its operations to be in a single transaction can call ensureTransactionStarted, and it's ok for this to be called repeatedly at mutiple levels of the stack. Then MainDriver calls commitTransactionIfAny at the end of every session (unless an exception is thrown). The result is that transactions can be begun at any level of the stack, and will be committed at the appropriate time. This flexibility allows code like SequoiaDriver to remain agnostic about transactions, since that driver doesn't actually need them for its own sake; but MainDriver needs a transaction for refurbish, and it can establish one with no cooperation from SequoiaDriver. Conversely, PandoDriver will need transactions for almost every operation, and it can establish them too; and yet later, if PandoDriver finds ways to skip transactions in certain cases, it has the freedom to do so.
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This allows the initial Pando implementation commit to show all the changes between the two.
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Implements #2.