Count data cells with stream-id 0 as delivered for SENDME purposes
From discussions in #8093 (moved), I made a branch called "not_8093" which looks like a fine idea to merge.
Its commit message was:
count DATA cells with stream ID 0 as delivered for SENDME purposes
Found while investigating 8093, but probably not the cause of it, since this bug would result in us sending too few SENDMEs, not in us receiving SENDMEs unexpectedly.
Bugfix on the fix for 7889, which has appeared in 0.2.4.10-alpha, but not yet in any released 0.2.3.x version.