LevelDB block and coin databases

Split off CBlockTreeDB and CCoinsViewDB into txdb-*.{cpp,h} files,
implemented by either LevelDB or BDB.

Based on code from earlier commits by Mike Hearn in his leveldb
branch.

src/makefile.linux-mingw

@@ -91,10 +91,12 @@ ifdef USE_LEVELDB
 LIBS += $(CURDIR)/leveldb/libleveldb.a $(CURDIR)/leveldb/libmemenv.a
 DEFS += -I"$(CURDIR)/leveldb/include" -DUSE_LEVELDB
 DEFS += -I"$(CURDIR)/leveldb/helpers"
-OBJS += obj/leveldb.o
+OBJS += obj/leveldb.o obj/txdb-leveldb.o
 leveldb/libleveldb.a:
 	@echo "Building LevelDB ..."; cd leveldb; TARGET_OS=OS_WINDOWS_CROSSCOMPILE CXXFLAGS="-I$(INCLUDEPATHS)" LDFLAGS="-L$(LIBPATHS)" make libleveldb.a libmemenv.a; cd ..
 obj/leveldb.o: leveldb/libleveldb.a
+else
+OBJS += obj/txdb-bdb.o
 endif
 
 obj/build.h: FORCE