Now we are able to weave 'add_source' into the existing index. This brings 'bzr pack' time down to ~23.6s (with debugging on). According to lsprof time for 'add_delta_source' overall dropped from ~5s down to about 300ms, and now the time for 'add_source' dropped 5s->3.3s->1.6s. Next thing is to probably bump the number of free slots.