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.mingw

@@ -94,10 +94,12 @@ ifdef USE_LEVELDB
 LIBS += $(CURDIR)/leveldb/libleveldb.a $(CURDIR)/leveldb/libmemenv.a
 DEFS += $(addprefix -I,$(CURDIR)/leveldb/include) -DUSE_LEVELDB
 DEFS += $(addprefix -I,$(CURDIR)/leveldb/helpers)
-OBJS += obj/leveldb.o
+OBJS += obj/leveldb.o obj/txdb-leveldb.o
 leveldb/libleveldb.a:
     cd leveldb; make libleveldb.a libmemenv.a; cd ..
 obj/leveldb.o: leveldb/libleveldb.lib
+else
+OBJS += obj/txdb-bdb.o
 endif
 
 obj/%.o: %.cpp $(HEADERS)