Remove LatestMap, to reduce memory allocation

[bboreham]

Previously, classes like StringLatestMap were implemented as a shim over LatestMap.
This meant that the memory allocations were:
[ps.Map leaf] -> [interface{} indirection] -> LatestEntry -> [interface{} indirection] -> [string contents]

Removing LatestMap and storing the entry via pointer rather than value gives us instead:

[ps.Map leaf] -> stringLatestEntry -> [string contents]

Also the LatestEntry was copied one more time on entry into the map. Thus this PR removes 50% of the memory allocations needed to create this particular structure, per entry.

(not counting the allocation for the map key)

[bboreham]

Some stats (run on a slightly different commit):

Running the following test:

package report                                                                                                                                                        
                                                                                                                                                                      
import "github.com/ugorji/go/codec"                                                                                                                                   
import "io/ioutil"                                                                                                                                                    
import "testing"

func TestDecode(t *testing.T) {
    b, _ := ioutil.ReadFile("fixed.msgpack")
    for i := 0; i < 100; i++ {
        r := Report{}
        handler := &codec.MsgpackHandle{}
        d := codec.NewDecoderBytes(b, handler)
        d.Decode(&r)
    }
}

where fixed.msgpack is a 1.5MB file supplied by Adam;

On master, commit d64d66e:

$ go test -run TestDecode
ok  	github.com/weaveworks/scope/report	1.935s
ok  	github.com/weaveworks/scope/report	1.989s
ok  	github.com/weaveworks/scope/report	1.937s
ok  	github.com/weaveworks/scope/report	2.065s

With this PR, commit de97a72:

PASS
ok  	github.com/weaveworks/scope/report	1.678s
ok  	github.com/weaveworks/scope/report	1.652s
ok  	github.com/weaveworks/scope/report	1.756s
ok  	github.com/weaveworks/scope/report	1.743s

That's about a 15% speed-up, fastest to fastest.

[2opremio]

Did you find generate_latest_map useful?

[2opremio]

(i.e. we don't really need to maintain it, so I hope it was helpful and you didn't update it because it was there)

[bboreham]

Yes, it's easier to edit the code once and generate twice, than to edit it twice.