tool: fix up wal dump, add wal dump-merged

[jbowens]

Adjust the wal dump tool to error on unrecognized key kinds, and surface errors
encounted more prominently in the output.

Additionally, add a new wal dump-merged command that dumps the merged view of
a WAL that's composed of multiple physical segment files.

Informs #3865.

[cockroach-teamcity]

This change is 

[RaduBerinde]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @aadityasondhi)

[jbowens]

I ended up expanding this while debugging #3865 so that I could reproduce #3865 using the debug tool.

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @aadityasondhi and @RaduBerinde)

[RaduBerinde]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @aadityasondhi and @jbowens)

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tool/wal.go line 209 at r2 (raw file):

	errf := func(offset wal.Offset, format string, args ...interface{}) {
		prefix := fmt.Sprintf("%s: error: ", offset)
		fmt.Fprintf(stderr, prefix+format+"\n", args...)

This kind of non-constant format string can trip up newer versions of lint checks (see cockroachdb/cockroach#129208).

I'd just pass an error and use errors.Wrap at the callsite and here to add info.

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tool/wal.go line 289 at r2 (raw file):

			s, err := rangekey.Decode(ik, value, nil)
			if err != nil {
				errf("%s: error decoding %s", w.fmtKey.fn(ukey), err)

[nit] errf prints to stdout with a newline, unlike the other cases here. Maybe keep the fmt.Fprintf(stdout here and remove it from errf.

If it's not hard, it would be nice to have this case show up in the test.

[jbowens]

Reviewable status: 0 of 2 files reviewed, all discussions resolved (waiting on @aadityasondhi and @RaduBerinde)

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tool/wal.go line 209 at r2 (raw file):

Previously, RaduBerinde wrote…

This kind of non-constant format string can trip up newer versions of lint checks (see cockroachdb/cockroach#129208).

I'd just pass an error and use errors.Wrap at the callsite and here to add info.

Done

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tool/wal.go line 289 at r2 (raw file):

Previously, RaduBerinde wrote…

[nit] errf prints to stdout with a newline, unlike the other cases here. Maybe keep the fmt.Fprintf(stdout here and remove it from errf.

If it's not hard, it would be nice to have this case show up in the test.

Done

I gave the test case a stab but aborted for now because it is tricky