Move CWalletDB code to new walletdb module.

In addition to standard code separation, this change opens the door
to fixing several include inter-dependencies.

bitcoin-qt.pro

@@ -117,6 +117,7 @@ HEADERS += src/qt/bitcoingui.h \
     src/net.h \
     src/key.h \
     src/db.h \
+    src/walletdb.h \
     src/script.h \
     src/init.h \
     src/irc.h \
@@ -181,6 +182,7 @@ SOURCES += src/qt/bitcoin.cpp src/qt/bitcoingui.cpp \
     src/checkpoints.cpp \
     src/addrman.cpp \
     src/db.cpp \
+    src/walletdb.cpp \
     src/json/json_spirit_writer.cpp \
     src/json/json_spirit_value.cpp \
     src/json/json_spirit_reader.cpp \