Add stream consumer group lag tracking and reporting (#9127)
Adds the ability to track the lag of a consumer group (CG), that is, the number of entries yet-to-be-delivered from the stream. The proposed constant-time solution is in the spirit of "best-effort." Partially addresses #8737. ## Description of approach We add a new "entries_added" property to the stream. This starts at 0 for a new stream and is incremented by 1 with every `XADD`. It is essentially an all-time counter of the entries added to the stream. Given the stream's length and this counter value, we can trivially find the logical "entries_added" counter of the first ID if and only if the stream is contiguous. A fragmented stream contains one or more tombstones generated by `XDEL`s. The new "xdel_max_id" stream property tracks the latest tombstone. The CG also tracks its last delivered ID's as an "entries_read" counter and increments it independently when delivering new messages, unless the this read counter is inv...
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