Motivated by garbage profiling in HTTP benchmarks. This
changes means new empty maps are just one small allocation
(the HMap) instead the HMap + the relatively larger h->buckets
allocation. This helps maps which remain empty throughout
their life.

benchmark               old ns/op    new ns/op    delta
BenchmarkNewEmptyMap          196          107  -45.41%

benchmark              old allocs   new allocs    delta
BenchmarkNewEmptyMap            2            1  -50.00%

benchmark               old bytes    new bytes    delta
BenchmarkNewEmptyMap          195           50  -74.36%

R=khr, golang-dev, r
CC=golang-dev
https://golang.org/cl/7722046