Make lsn_reset ("detach databases") optional and off by default.

Add an option -detachdb (and entry in OptionDialog), without which no
lsn_reset is called on addr.dat and blkindex.dat. That means these
files cannot be moved to a new environment, but shutdown can be
significantly faster. The wallet file is always lsn_reset'ed.

-detachdb corresponds to the old behaviour, though it is off by
default now to speed up shutdowns.

src/qt/optionsmodel.h

@@ -26,7 +26,8 @@ public:
         Fee, // qint64
         DisplayUnit, // BitcoinUnits::Unit
         DisplayAddresses, // bool
-        OptionIDRowCount
+        DetachDatabases, // bool
+        OptionIDRowCount,
     };
 
     void Init();