Add debug command to "simulate" GC run
Add debug command to "simulate" GC run

`./cockroach estimate-gc <dir> [range_id]`

The output format is

```
RangeID: 92 [/Table/53/2/"k\xcaPe\xce\xe6I\xbc\x8af\xd9\xf8V[L\xdb"/Tue Mar 15 17:24:18 UTC 2016, /Max):
storage.GCInfo{
    Now:                        roachpb.Timestamp{WallTime:1458267071598809847, Logical:0},
    Policy:                     config.GCPolicy{TTLSeconds:86400},
    GCKeys:                     22448,
    IntentsConsidered:          0,
    IntentTxns:                 0,
    TransactionSpanTotal:       38,
    TransactionSpanGCAborted:   23,
    TransactionSpanGCCommitted: 15,
    TransactionSpanGCPending:   0,
    TransactionSpanGCNum:       38,
    SequenceSpanTotal:          65026,
    SequenceSpanConsidered:     65026,
    SequenceSpanGCNum:          0,
    PushTxn:                    63691,
    ResolveTotal:               170,
    ResolveSuccess:             170,
}
```

The alert observer will note that this indicates a horrible upcoming GC
performance, in particular regarding the sequence span and the associated
pushes.

maxlang/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,nieyy/cockroach,petermattis/cockroach,sunya123/cockroach,arjun4084346/cockroach,glycerine/cockroach,paperstreet/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,nieyy/cockroach,veteranlu/cockroach,sunya123/cockroach,vivekmenezes/cockroach,maxlang/cockroach,maxlang/cockroach,arjun4084346/cockroach,nieyy/cockroach,petermattis/cockroach,veteranlu/cockroach,sunya123/cockroach,maxlang/cockroach,bg451/cockroach,glycerine/cockroach,petermattis/cockroach,veteranlu/cockroach,nieyy/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,glycerine/cockroach,paperstreet/cockroach,cockroachdb/cockroach,nieyy/cockroach,sunya123/cockroach,glycerine/cockroach,maxlang/cockroach,vivekmenezes/cockroach,bg451/cockroach,sunya123/cockroach,vaibhavarora/cockroach,bg451/cockroach,bg451/cockroach,paperstreet/cockroach,petermattis/cockroach,sunya123/cockroach,nieyy/cockroach,glycerine/cockroach,bg451/cockroach,nieyy/cockroach,veteranlu/cockroach,petermattis/cockroach,paperstreet/cockroach,arjun4084346/cockroach,maxlang/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,glycerine/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,veteranlu/cockroach,paperstreet/cockroach,bg451/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,sunya123/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,glycerine/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,nieyy/cockroach,arjun4084346/cockroach,petermattis/cockroach,petermattis/cockroach,cockroachdb/cockroach,maxlang/cockroach,bg451/cockroach,petermattis/cockroach,arjun4084346/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,veteranlu/cockroach,cockroachdb/cockroach,glycerine/cockroach,sunya123/cockroach,cockroachdb/cockroach,paperstreet/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,maxlang/cockroach