Prepare TransactionManager for multi-repo support

TransactionManager needs to support multiple repositories in a partition in order to accommodate for object pools and their member repos. As we've so far been operating on single repository per partition, the TransactionManager contains multiple locations where we assume we're operating on a single repository. This MR removes the assumption from those locations, and ends up in a state where relative path of the repository a transaction is operating is stored in the transaction, and rest of the location in TransactionManager use that path. It will be easy to adapt the public interface after this MR to take in the relative path of the target repository in Begin().