code style: In ChronoSync2013, rename "co" to "data". Refs #2199.
code style: In ChronoSync2013, rename "usrseq_" to "sequenceNo_". Refs #2199.
code style: In ChronoSync2013, etc. fix spelling of variables, e.g. "inst" to "interest" and "digest_tree" to "digestTree". Refs #2199.
Merge branch 'issue/2199-sync-variables-code-style'. Refs #2199.
http://redmine.named-data.net/issues/2199