Optimize compactor to use multi-IO API

This updates couch_db_updater to use the new multi-IO API functions
(append_terms/pread_terms) in couch_file. This optimization benefits us
by no longer requiring the `couch_emsort:merge/1` step to copy
`#full_doc_info{}` records multiple times while also not being penalized
by signficantly increasing the number of calls through couch_file APIs.
1 file changed