Postgres fixes (#84)

* Postgres fixes

* Create partial index for ledger_diffs, to avoid indexing the first
  ledger's objects

* Don't write duplicate keys to successor table

* default to 4 markers when syncing cache

* remove isFirst from writeLedger interface

src/backend/BackendInterface.h

@@ -19,7 +19,6 @@ class DatabaseTimeout : public std::exception
 class BackendInterface
 {
 protected:
-    bool isFirst_ = true;
     std::optional<LedgerRange> range;
     SimpleCache cache_;
 
@@ -153,8 +152,7 @@ public:
     virtual void
     writeLedger(
         ripple::LedgerInfo const& ledgerInfo,
-        std::string&& ledgerHeader,
-        bool isFirst = false) = 0;
+        std::string&& ledgerHeader) = 0;
 
     void
     writeLedgerObject(std::string&& key, uint32_t seq, std::string&& blob);