util/chunk: preallocate the columns array in MutRow to avoid array resize

[kennytm]

What problem does this PR solve?

When checking the flamegraph of Lightning (which does lots of INSERTs) on a table with 37 columns, I found that MutRowFromDatums spends a noticeable amount of time in runtime.growslice. This means the capacity of c.columns isn't large enough and append() will need to reallocate and move the content (as of Go 1.11 the capacity grows like 0 → 2 → 4 → 8 → 16 → …).

What is changed and how it works?

Since the final length is known, we just preallocate the slice with the known capacity.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  • Patched the TiDB dependency in Lightning and verified that runtime.growslice no longer appears directly inside MutRowFromDatums in the pprof CPU profile.

Code changes

Side effects

Related changes

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This change is 

[kennytm]

/run-all-tests

[eurekaka]

LGTM

[zz-jason]

LGTM