Add transactions

[jpsamaroo]

At the moment, our storage layer (in StorageState) is non-transactional, and so it's not possible to ensure that a certain set of operations occur without other (incompatible or performance-degrading) operations occurring in-between. This can make it impossible for the SimpleRecencyAllocator (which provides our swap-to-disk functionality) to be able to provide any guarantees that memory/disk usage actually falls within user-provided limits.

The obvious solution here is to add "transactions", where the set of operations to be performed in sequence are batched up and submitted as a single logical operation. Any transaction in progress can exclude other operations or transactions from occurring on the same piece(s) of data until the current transaction completes. Additionally, for performance, we can implement some logic to "merge" compatible operations across multiple transactions (or within the same transaction) when the end result will be identical to running the operations exclusively.

@dbedi3311