stats: Fix rates inconsistency bug.
vtgate: add multidimensional QPS counters.
stats: rename Map* to Multi* to make multidimensionality of the counters more obvious.

Conflicts:
	go/cmd/vtgate/vtgate.go
	go/vt/tabletserver/schema_info.go
	go/vt/vtgate/scatter_conn.go
QPS and error rates in vtgate's status.
Merge branch 'master' of github.com:youtube/vitess