TransactionManager needs to assign globally unique IDs when multiple active masters

With the current transaction ID generation, if multiple active TransactionManagers are running, we will wind up with conflicts between the transaction IDs assigned by each active instance. We need to provide a mechanism for each instance to assign globally unique IDs. Something like the snowflake algorithm would work, where we reserve a number of bits for an instance ID. Global ordering of the transaction IDs should not matter, if we partition the update key space across transaction manager, such that all updates within a given partition go through the same manager instance.