Synchronize transaction beginning with log application

Begin is called to start a new transaction. Transaction should see all data that was committed prior to the transaction beginning. As it is currently, Begin has no synchronization with the log entry application. It's possible that a transaction begins before all committed data has been applied to the repository and thus available for the transaction to read. This commit addresses the problem by waiting for committed data to be applied before the transaction can begin.

When Begin is called, it determines the transaction's read index. The read index is a log index at which the data is to be read at. When a transaction begins, we thus have to ensure all of the data logged prior to the transaction starting has been applied to the repository. This is achieved by the TransactionManager maintaining notification channels which it broadcasts log entry application to all waiters. These channels are listened on in Begin, and used to block the transaction from beginning prior to the committed data being applied. This guarantees the transaction will read all data committed prior to its start.

As the Begin now waits for all data to be available, we no longer have to wait for a log entry to be applied prior to releasing a Commit call. The writer will is guaranteed to see the data it wrote when it begins a new transaction. Tests are updated to reflect this.

Part of #4792 (closed)