Improve blocking borrow logic in pool

[robsdedude]

• After waiting for room in the pool, borrow also considers creating a new connection (if possible)
• After waiting for room in the pool, borrow reads (but doesn't update) the routing table to get more up-to-date routing info
• Connections are always fully returned to and then borrowed from the pool. This means the health check and other initializers are guaranteed to be run.