Rework deferred node logic and async fetch behavior

This comment explains this patch and the associated patches
that should be folded into it. This paragraph should be removed
when the patches are folded after review.

This change significantly improves ledger sync and fetch
times while reducing memory consumption. The change affects
the code from that begins with SHAMap::getMissingNodes and runs
through to Database::threadEntry.

The existing code issues a number of async fetches which are then
handed off to the Database's pool of read threads to execute.
The results of each read are placed in the Database's positive
and negative caches. The caller waits for all reads to complete
and then retrieves the results out of these caches.

Among other issues, this means that the results of the first read
cannot be processed until the last read completes. Additionally,
all the results must sit in memory.

This patch changes the behavior so that each read operation has a
completion handler associated with it. The completion of the read
calls the handler, allowing the results of each read to be
processed as it completes. As this was the only reason the
negative and positive caches were needed, they can now be removed.

The read generation code is also no longer needed and is removed.
The batch fetch logic was never implemented or supported and is
removed.

src/ripple/core/impl/Config.cpp

@@ -41,7 +41,7 @@
 namespace ripple {
 
 // The configurable node sizes are "tiny", "small", "medium", "large", "huge"
-inline constexpr std::array<std::pair<SizedItem, std::array<int, 5>>, 13>
+inline constexpr std::array<std::pair<SizedItem, std::array<int, 5>>, 11>
     sizedItems{{
         // FIXME: We should document each of these items, explaining exactly
         // what
@@ -53,8 +53,6 @@ inline constexpr std::array<std::pair<SizedItem, std::array<int, 5>>, 13>
         {SizedItem::ledgerSize, {{32, 128, 256, 384, 768}}},
         {SizedItem::ledgerAge, {{30, 90, 180, 240, 900}}},
         {SizedItem::ledgerFetch, {{2, 3, 4, 5, 8}}},
-        {SizedItem::nodeCacheSize, {{16384, 32768, 131072, 262144, 524288}}},
-        {SizedItem::nodeCacheAge, {{60, 90, 120, 900, 1800}}},
         {SizedItem::hashNodeDBCache, {{4, 12, 24, 64, 128}}},
         {SizedItem::txnDBCache, {{4, 12, 24, 64, 128}}},
         {SizedItem::lgrDBCache, {{4, 8, 16, 32, 128}}},