colblk: check first timestamp before binary search

This commit improves the code in the cockroach key seeker to first
check the latest timestamp (which will be the one we're looking for in
a majority of cases).

```
name                                                                                                                 old time/op    new time/op    delta
CockroachDataBlockIterShort/AlphaLen=8,Prefix=8,Shared=4,KeysPerPrefix=4,Logical=10,ValueLen=8/Next-10                 11.4ns ± 4%    11.2ns ± 1%    ~     (p=0.308 n=8+8)
CockroachDataBlockIterShort/AlphaLen=8,Prefix=8,Shared=4,KeysPerPrefix=4,Logical=10,ValueLen=8/SeekGE-10                118ns ± 0%     118ns ± 0%  +0.24%  (p=0.023 n=8+8)
CockroachDataBlockIterShort/AlphaLen=8,Prefix=8,Shared=4,KeysPerPrefix=4,Logical=10,ValueLen=8/SeekGELatest-10          109ns ± 0%     108ns ± 6%  -1.29%  (p=0.019 n=8+7)
CockroachDataBlockIterShort/AlphaLen=8,Prefix=128,Shared=64,KeysPerPrefix=4,Logical=10,ValueLen=128/Next-10            10.6ns ± 0%    10.6ns ± 0%  -0.64%  (p=0.000 n=7+8)
CockroachDataBlockIterShort/AlphaLen=8,Prefix=128,Shared=64,KeysPerPrefix=4,Logical=10,ValueLen=128/SeekGE-10          95.0ns ± 0%    92.2ns ± 0%  -2.94%  (p=0.000 n=8+8)
CockroachDataBlockIterShort/AlphaLen=8,Prefix=128,Shared=64,KeysPerPrefix=4,Logical=10,ValueLen=128/SeekGELatest-10    88.2ns ± 0%    83.6ns ± 0%  -5.23%  (p=0.000 n=8+7)
````

sstable/colblk/cockroach_test.go

@@ -287,11 +287,18 @@ func (ks *cockroachKeySeeker) seekGEOnSuffix(index int, seekSuffix []byte) (row
 
  // TODO(jackson): What if the row has an untyped suffix?
 
- // Binary search between [index, prefixChanged.SeekSetBitGE(index+1)].
+ // First check the suffix at index, because querying for the latest value is
+ // the most common case.
+ if latestWallTime := ks.mvccWallTimes.At(index); latestWallTime < seekWallTime ||
+ (latestWallTime == seekWallTime && uint32(ks.mvccLogical.At(index)) <= seekLogicalTime) {
+ return index
+ }
+
+ // Binary search between [index+1, prefixChanged.SeekSetBitGE(index+1)].
  //
  // Define f(i) = true iff key at i is >= seek key.
  // Invariant: f(l-1) == false, f(u) == true.
- l := index
+ l := index + 1
  u := ks.reader.prefixChanged.SeekSetBitGE(index + 1)
  for l < u {
  h := int(uint(l+u) >> 1) // avoid overflow when computing h