Fix advisory delete affect on history acquisition (RIPD-1112):

* Revert 0efb929898
* Advisory delete setting of 0 (never) does not affect history fetching

The previous commit addressing RIPD-1112 could interact with
advisory delete and cause some history not to be acquired even
configured to acquire. This reverts that commit and provides
a better fix.

The advisory delete setting protects ledgers from being
removed by online delete by exempting them until they are
approved for purge by administrative command. However, not
connecting this with history acquisition could cause new
ledgers in the protected range not to be acquired if the
server loses sync.

With this change, the default advisory delete setting, zero (never)
causes the regular server history setting to control the acquisition
of history. Setting advisory delete to a value greater than zero,
if advisory delete is enabled, will cause the server to fetch and
maintain history back to that point.

This should produce sane behavior across server restarts, losses of
sync, and so on. You can no longer use the "hack" of setting
advisory delete to zero to tell the server to fetch and keep as much
history as possible, but you can achieve the same effect by setting
it to one.

src/ripple/app/ledger/impl/LedgerMaster.cpp

@@ -1560,12 +1560,15 @@ LedgerMaster::shouldAcquire (
     std::uint32_t const ledgerHistoryIndex,
     std::uint32_t const candidateLedger) const
 {
-    // If the ledger could be the current ledger acquire it.
-    // Otherwise, acquire it only if it's within our range of
-    // desired history and we wouldn't delete it if we had it.
-    bool ret = (candidateLedger >= currentLedger) ||
-        ((candidateLedger >= ledgerHistoryIndex) &&
-        ((currentLedger - candidateLedger) <= ledgerHistory));
+
+    // Fetch ledger if it might be the current ledger,
+    // is requested by the advisory delete setting, or
+    // is within our configured history range
+
+    bool ret (candidateLedger >= currentLedger ||
+        ((ledgerHistoryIndex > 0) &&
+            (candidateLedger > ledgerHistoryIndex)) ||
+        (currentLedger - candidateLedger) <= ledgerHistory);
 
     JLOG (m_journal.trace())
         << "Missing ledger "

src/ripple/app/misc/SHAMapStoreImp.h

@@ -150,12 +150,16 @@ public:
         return setup_.advisoryDelete;
     }
 
+    // All ledgers prior to this one are eligible
+    // for deletion in the next rotation
     LedgerIndex
     getLastRotated() override
     {
         return state_db_.getState().lastRotated;
     }
 
+    // All ledgers before and including this are unprotected
+    // and online delete may delete them if appropriate
     LedgerIndex
     getCanDelete() override
     {