Refactor summaries of visor

[jdknives]

Original comment from @i-hate-nicknames :

There are multiple levels of extra that seems to have been added over time. Visor has summary and extra summary, and hypervisor has his own extra summary, plus response wrappers around visor summaries:
hypervisor provides:

1. visor.Summary (via getVisor()), wraps in summaryResp
2. visor.ExtraSummary (via getVisorSummary()), wraps in extraSummaryResp
3. its own extra summary (extra-extra now?) (via getVisorsExtraSummary()), extraSummaryWithDmsgResp
   There are some overlaps between those structures but not always, e.g. visor.ExtraSummary Dmsg is different from Dmsg in extraSummaryWithDmsgResp. So we cannot simply embed one into other.

I would propose:

1. Remove Dmsg from visor.ExtraSummary. This field is not populated by visor api, so there is no reason to have it there.
2. Investigate why do we need dmsg stats for all visors in getVisorSummary, since we are asking for stats of a single visor.
   In case we don't need that data, extraSummaryWithDmsgResp and extraSummaryResp become equivalent and the former
   can be removed
3. Factor out TCPAddr and Online fields into its own struct, maybe smth like NetworkStats?
4. Rename summary to overview like @jdknives suggested, and extra summary to just summary