fix: repair build against renamed namespaces from develop

develop renamed CamelCase namespaces to snake_case with no compatibility
aliases, so lines this branch added still referenced the old names. The
merge was textually clean because develop never touched those lines, which
left the breakage invisible to git and visible only at compile time.

  PeerFinder:: -> peer_finder::   Overlay.h, OverlayImpl.h, MetricMacros.cpp
  Tuning::     -> tuning::        PeerImp.cpp (the other call sites in this
                                  file already used the lowercase form)

Two further merge artifacts:

  json_value.h  develop added `using value_type = Value const` to
                ValueConstIterator, and this branch had added
                `using value_type = Value` for the same reason, so the merge
                kept both and they redefined the alias with different types.
                Keep develop's const-qualified type -- correct for an
                iterator whose reference is `Value const&` -- and keep only
                this branch's iterator_category addition.

  InboundLedger.h  <string> is no longer used directly by the header, but
                   five translation units that include it use std::string
                   without including <string> themselves. Mark it
                   `IWYU pragma: keep` rather than remove it, matching
                   suite_list.h.

include/xrpl/json/json_value.h

@@ -629,7 +629,6 @@ public:
     // this as a Cpp17InputIterator; without them it defaults to output-only,
     // which breaks standard algorithms (e.g. std::all_of). The iterator walks a
     // map both ways via ++/--, so it is bidirectional.
-    using value_type = Value;
     using iterator_category = std::bidirectional_iterator_tag;
     using reference = Value const&;
     using pointer = Value const*;