read-committed and read-uncommitted transactions can stall eviction

Summary:

Fix an issue where eviction stalls because the thread pinning the oldest transaction ID in the system has been co-opted to assist with eviction and is preventing the system's oldest transaction ID from moving forward (which would release pages for eviction).

Read-committed and read-uncommitted transactions enter a transaction ID into the global table to stop updates they're reading from being freed, and that can prevent the oldest ID from moving forward, stalling eviction.