Support running Gitaly with transactions enabled

This MR adds a configuration option for running Gitaly with transaction management enabled. While transactions are not yet ready for production use, this allows them to be configured for development and testing purposes.

Transactions are integrated with rest of Gitaly by configuring an interceptor in GitalyServerFactory that handles manages beginning, committing and rolling back transactions for RPCs.

PartitionManager is initialized on Gitaly's start up, and closed when Gitaly is terminating. Initializing and stopping the partition manager opens and closes the database handles for each storage where Gitaly stores some transaction processing related state.