Convert Cayley indexing to an append-only log

Fixes #70

In order to provide for replication, graph history and other neat clustering features, I created the interface `QuadWriter` which can take care of all the replication logic; TripleStore (soon, QuadStore, I'm guessing) should store the things that have been accepted. Further, it should be kept as a log -- a follow-up CL which adds `GetRange(from, to int64) []*graph.Delta` to the TripleStore interface is forthcoming.

Some question of moving, renaming and ownership is certainly valid.

This also turns Cayley into a full quadstore for all backends, which it was heading toward anyway. This PR finalizes that.

Anyway, this means that nothing really gets deleted, but that indexed quads get extra data about when they're valid, and the triplestore^Hquadstore iterators should check the validity. This adds some amount of slowdown for Next(), but  how much?

```
$ benchcmp nolog.txt prelog.txt
benchmark                                   old ns/op       new ns/op       delta
BenchmarkNamePredicate                      1075967         1144500         +6.37%
BenchmarkLargeSetsNoIntersection            48881903        56925194        +16.45%
BenchmarkVeryLargeSetsSmallIntersection     509579408       757046655       +48.56%
BenchmarkHelplessContainsChecker            26454974514     27333222110     +3.32%
BenchmarkNetAndSpeed                        15422744        16408261        +6.39%
BenchmarkKeanuAndNet                        12878175        13006483        +1.00%
BenchmarkKeanuAndSpeed                      14542854        14950108        +2.80%
BenchmarkKeanuOther                         61897336        76940891        +24.30%
BenchmarkKeanuBullockOther                  78667479        81221650        +3.25%
```

A couple percent, but not noise. Incidentally, this is exacerbated by the Materialize iterator that recently went in; it does a lot of Next()ing, so on things where it aborts (HelplessContainsChecker) it doesn't have as problematic an impact. So, the wins from Materialize are still large, just less so now.

So what's the overhead on queries going through the addition/deletion/readdition cycle, other than using more persistence space? This now gets tested.

The answer to that, at least with the memstore is:
```
$ benchcmp prelog.txt postlog.txt
benchmark                                   old ns/op       new ns/op       delta
BenchmarkNamePredicate                      1144500         1108439         -3.15%
BenchmarkLargeSetsNoIntersection            56925194        57843995        +1.61%
BenchmarkVeryLargeSetsSmallIntersection     757046655       747045447       -1.32%
BenchmarkHelplessContainsChecker            27333222110     28253011444     +3.37%
BenchmarkNetAndSpeed                        16408261        17083855        +4.12%
BenchmarkKeanuAndNet                        13006483        13495442        +3.76%
BenchmarkKeanuAndSpeed                      14950108        15693748        +4.97%
BenchmarkKeanuOther                         76940891        68439409        -11.05%
BenchmarkKeanuBullockOther                  81221650        74245237        -8.59%
```

Meh. Not too much.

Obviously it'll be a bit slower for larger backends, but it'll still be fairly small because it fails fairly fast; we don't have to go through the whole tree for things that are invalid, we just go to the next thing in the underlying persistence iterator.

A migration tool to collapse the history (ie, only keep additions of things that are valid) might make sense in the future, but should be a separate tool.

/cc @kortschak @pbnjay


Do you mind if we leave these as `it` (and below). It makes the godoc read better.