add constants for shared (GUI/core) -dbcache settings

- adds nDefaultDbCache, nMaxDbCache and nMinDbCache in txdb.h

src/qt/forms/optionsdialog.ui

@@ -53,17 +53,7 @@
           </widget>
          </item>
          <item>
-          <widget class="QSpinBox" name="databaseCache">
-           <property name="toolTip">
-            <string>Set database cache size in megabytes (default: 25)</string>
-           </property>
-           <property name="maximum">
-            <number>1024</number>
-           </property>
-           <property name="value">
-            <number>25</number>
-           </property>
-          </widget>
+          <widget class="QSpinBox" name="databaseCache"/>
          </item>
          <item>
           <widget class="QLabel" name="databaseCacheUnitLabel">

src/qt/optionsdialog.cpp

@@ -14,8 +14,9 @@
 #include "monitoreddatamapper.h"
 #include "optionsmodel.h"
 
+#include "main.h" // for CTransaction::nMinTxFee
 #include "netbase.h"
-#include "main.h"
+#include "txdb.h" // for -dbcache defaults
 
 #include <QDir>
 #include <QIntValidator>
@@ -34,7 +35,8 @@ OptionsDialog::OptionsDialog(QWidget *parent) :
     GUIUtil::restoreWindowGeometry("nOptionsDialogWindow", this->size(), this);
 
     /* Main elements init */
-    ui->databaseCache->setMaximum(sizeof(void*) > 4 ? 4096 : 1024);
+    ui->databaseCache->setMinimum(nMinDbCache);
+    ui->databaseCache->setMaximum(nMaxDbCache);
 
     /* Network elements init */
 #ifndef USE_UPNP

src/qt/optionsmodel.cpp

@@ -14,6 +14,7 @@
 #include "init.h"
 #include "main.h"
 #include "net.h"
+#include "txdb.h" // for -dbcache defaults
 #ifdef ENABLE_WALLET
 #include "wallet.h"
 #include "walletdb.h"
@@ -84,7 +85,7 @@ void OptionsModel::Init()
 #endif
 
     if (!settings.contains("nDatabaseCache"))
-        settings.setValue("nDatabaseCache", 100);
+        settings.setValue("nDatabaseCache", nDefaultDbCache);
     if (!SoftSetArg("-dbcache", settings.value("nDatabaseCache").toString().toStdString()))
         strOverriddenByCommandLine += "-dbcache ";