Fix node auto-configuration code:

The `node_size` configuration option is used to automatically
configure various parameters (cache sizes, timeouts, etc) for
the server.

A previous commit included changes that caused incorrect values
to be returned which can result in sub-optimal performance that
can manifest as difficulty syncing to the network, or increased
disk I/O and/or memory usage. The problem was introduced with
commit 66fad62e66.

This commit, if merged, fixes the code to ensure that the correct
values are returned and introduces a compile-time check to prevent
this issue from reoccurring.

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

@@ -152,7 +152,7 @@ LedgerMaster::LedgerMaster (Application& app, Stopwatch& stopwatch,
     , fetch_depth_ (app_.getSHAMapStore ().clampFetchDepth (
         app_.config().FETCH_DEPTH))
     , ledger_history_ (app_.config().LEDGER_HISTORY)
-    , ledger_fetch_size_ (app_.config().getSize (siLedgerFetch))
+    , ledger_fetch_size_ (app_.config().getValueFor(SizedItem::ledgerFetch))
     , fetch_packs_ ("FetchPack", 65536, std::chrono::seconds {45}, stopwatch,
         app_.journal("TaggedCache"))
 {