opt(builder): Use z.Allocator for building tables

[ahsanbarkati]

Avoid Go memory spikes by using z.Allocator to allocate blocks during table generation. Also avoid having to copy data over, unless necessary -- instead write the data directly to the mmapped file.

With these changes, we're able to load 1 billion randomly generated 32-byte keys, 128-byte values in 1.5 hrs, with memory usage maxing out to 5 GB.

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This change is 

[manishrjain]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @ahsanbarkati, @jarifibrahim, and @manishrjain)

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table/builder.go, line 251 at r1 (raw file):

	defer b.bufLock.Unlock()
	b.buf.AllocateOffset(int(sz))
	b.sz += sz

If you're seeing some issues, then maybe do an assert in a few places about b.sz == LenNoPadding. But, ideally, we shouldn't be using b.sz at all?

[codelingo]

1 issues found. 1 rules errored during the review.

[codelingo]

Initialize n using a composite literal to simplify the code. as specified in Effective Go

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[codelingo]

2 issues found. 1 rules errored during the review.

[jarifibrahim]

LGTM

[codelingo]

1 issues found. 1 rules errored during the review.

[codelingo]

Omit unnecessary else when if doesn't flow into the next statement. as specified in Effective Go

View Rule

[manishrjain]

Reviewed 5 of 13 files at r8, 1 of 2 files at r9, 4 of 7 files at r10, 9 of 9 files at r11.
Dismissed @codelingo[bot] from 3 discussions.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @ahsanbarkati)

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stream_writer.go, line 412 at r11 (raw file):

		data := builder.Finish()
		var err error
		if tbl, err = table.OpenInMemoryTable(data, fileID, builder.Opts()); err != nil {

might consider passing the builder over to OpenInMemoryTable.