InMemory transaction fixes.

[tachyonics]

Issue #, if available: #89

Description of changes: Fixes to InMemoryDynamoDBCompositePrimaryKeyTable for transactions

1. Perform all updates for the transaction atomically
2. If the transaction fails, revert the store to its state before the transaction
3. Throw the DuplicateItem error reason correctly if a row exists prior to the transaction
4. Throw the transactionConditionalCheckFailed correctly if a constraint fails.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[knovichikhin]

Does this work for concurrent transactions? Is this possible?

1. Transaction A takes a store snapshot
2. Transaction B takes a store + A snapshot
3. Transaction B succeeds
4. Transaction A fails and restores store snapshot without B.

[tachyonics]

This can't happen as we are executing everything on a serial DispatchQueue. Only one transaction will be executing on the dispatch queue so Transaction A will complete (either successfully or restoring the state) before Transaction B can be run.

https://github.com/amzn/smoke-dynamodb/blob/in_memory_transaction_fixes/Sources/SmokeDynamoDB/InMemoryDynamoDBCompositePrimaryKeyTableStore.swift#L74
https://developer.apple.com/documentation/dispatch/dispatchqueue/2300059-init

This effectively uses a global lock for the table which would be significantly less efficient than DynamoDB in the real world but should ensure correctness.