Allow multiple PENDING checkpoints

QMIN should not be limited to only having a single PENDING checkpoint. Absent some special considerations, the encoder wants to be able to partition the dynamic entry space into chunks of about equal size because it allows a checkpoint gracefully to age out later. A delayed ACK_FLUSH message from the decoder means that the number of entries in the NEW checkpoint will grow and grow. There is no reason why more than one PENDING checkpoint cannot be allowed, as both flushes and their ACKs are delivered in order and are not ambiguous.

No file or test is named. Start by locating checkpoint state handling in the QMIN implementation and review how ordered flushes and ACKs are represented; done means allowing multiple PENDING checkpoints without ambiguity while preserving ordered delivery.

Written by the indexing model from the issue text.