Fix XTRIM or XADD with LIMIT may delete more entries than Count.

[huangzhw]

The decision to stop trimming due to LIMIT in XADD and XTRIM was after the limit was reached.
i.e. the code was deleting at least that count of records (from the LIMIT argument's perspective, not the MAXLEN),
instead of up to that count of records.
see #9046

[oranagra]

so you're saying it's an "off by one" bug. i.e. checking the limit after deletion rather than before. right?

but note that what we documented is:

The ~ argument between the MAXLEN strategy and the threshold means that the user is requesting to trim the stream so its length is at least the threshold, but possibly slightly more

so are you sure there's a bug? or maybe we just lost our way...

[huangzhw]

Checking the limit is still before deletion. For example, limit is 10, deletion is 9, the lenght of next node to delete is 5. The old code will delete the 5 node, so deletion is 14. The new code will not delete the node, so deletion is still 9.

[oranagra]

ohh, never mind. i mixed the documentation of MAXLEN with the LIMIT feature (two different features)

[oranagra]

i've edited the top comment, let me know (or fix it), if wrong.

[huangzhw]

The old code might still delete less records than count, if the records to delete is less than required. So whether should make it more accurate.