Clear old Validations during online delete (RIPD-870):

* Add Validations.LedgerSeq and .InitialSeq fields.
* Clean up logging.
* Lower online delete minimum for standalone mode.
* Unit tests of online_delete.

src/ripple/app/misc/SHAMapStore.h

@@ -41,6 +41,7 @@ class SHAMapStore
 public:
     struct Setup
     {
+        bool standalone = false;
         std::uint32_t deleteInterval = 0;
         bool advisoryDelete = false;
         std::uint32_t ledgerHistory = 0;