Clear up some confusion about attribute count and external references

Now the count is always adjacent to whereever the key/value pair is
stored, and is not kept in the serial feature object other than as
the length of the vectors of keys and values.

plugin.cpp

@@ -393,7 +393,6 @@ serial_feature parse_feature(json_pull *jp, int z, unsigned x, unsigned y, std::
 			sf.bbox[0] = sf.bbox[1] = LLONG_MAX;
 			sf.bbox[2] = sf.bbox[3] = LLONG_MIN;
 			sf.extent = 0;
-			sf.m = 0;
 			sf.metapos = 0;
 			sf.has_id = false;