Cleanup the 'PeerSet' hierarchy:

This commit introduces no functional changes but cleans up the
code and shrinks the surface area by removing dead and unused
code, leveraging std:: alternatives to hand-rolled code and
improving comments and documentation.

.clang-format

@@ -18,7 +18,7 @@ AlwaysBreakBeforeMultilineStrings: true
 AlwaysBreakTemplateDeclarations: true
 BinPackArguments: false
 BinPackParameters: false
-BraceWrapping:   
+BraceWrapping:
   AfterClass:      true
   AfterControlStatement: true
   AfterEnum:       false
@@ -43,8 +43,8 @@ Cpp11BracedListStyle: true
 DerivePointerAlignment: false
 DisableFormat:   false
 ExperimentalAutoDetectBinPacking: false
-ForEachMacros:   [ foreach,   Q_FOREACH,  BOOST_FOREACH ]
-IncludeCategories: 
+ForEachMacros:   [ Q_FOREACH,  BOOST_FOREACH ]
+IncludeCategories:
   - Regex:           '^<(BeastConfig)'
     Priority:        0
   - Regex:           '^<(ripple)/'
@@ -84,4 +84,4 @@ SpacesInParentheses: false
 SpacesInSquareBrackets: false
 Standard:        Cpp11
 TabWidth:        8
-UseTab:          Never
+UseTab:          Never