this new option declares how many max inflight-or-idle open connections a session may hold. connections get recycled in case a new one is needed and the pool has closed connections to discard. the same pool timeout error applies as for max_connections_per_origin
the previous iteration relied on internal behaviour do delete the correct callback; in the process, logic to delete all callbacks from an interval was accidentally committed, which motivated this refactoring. the premise is: timeouts can cancel the timer; they set themselves as active until done; operation timeouts rely on the previous to be ignored or not.
a new error, OperationTimeoutError, was added for that effect
defaulting to unbounded, in order to preserve current behaviour; this will cap the number of connections initiated for a given origin for a pool, which if not shared, will be per-origin; this will include connections from separate option profiles
a pool timeout is defined to checkout a connection when limit is reeached
