Fix race condition in pool (#610)

* Fix race condition in pool

Co-authored-by: Robsdedude <rouven.bauer@neo4j.com>

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Co-authored-by: Robsdedude <rouven.bauer@neo4j.com>

neo4j/internal/pool/pool.go

@@ -415,6 +415,9 @@ func (p *Pool) Return(ctx context.Context, c idb.Connection) {
 
 	// Shouldn't return a too old or dead connection back to the pool
 	if !isAlive || age >= p.config.MaxConnectionLifetime {
+		// Fix for race condition where expired connections could be reused or closed concurrently.
+		// See: https://github.com/neo4j/neo4j-go-driver/issues/574
+		isAlive = false
 		p.unreg(ctx, serverName, c, now)
 		p.log.Infof(log.Pool, p.logId, "Unregistering dead or too old connection to %s", serverName)
 	}