db: add a "bytes" type, corresponding to ByteBuffer (serialized values)

Using sstring can lead to confusion with UTF8 strings.

The Java byte type is signed, so make bytes' internal type be signed as
well (even though Cassandra tries to treat it as unsigned).

While we should use int8_t, sstring is not perfectly compatible with this
yet, so add a FIXME and use char instead.

database.hh

@@ -19,6 +19,9 @@
 
 using bytes = basic_sstring<uint8_t, uint32_t, 31>;
 
+// FIXME: should be int8_t
+using bytes = basic_sstring<char, uint32_t, 31>;
+
 struct row {
     std::vector<boost::any> cells;
 };