Decrease tick interval to 20 ms.

[srh]

Note that etcd does 5 ms, and Cockroach does 200 ms by default. Note that Cockroach has complicated usage of the library so things there might not be so simple. I went with 20 ms because that's low enough to make us catch up a replica on the same order of time that an initial series of writes happened, on a particular piece of hardware with a particular usage pattern.

Connected to #1180.

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This change is 

[srh]

Edited the comments, which I should have looked at.

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[manishrjain]

@janardhan1993 just checked. The pings within Amazon DC are under a millisecond (200 micros). So, we should be good on that account as well.

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Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, 2 unresolved discussions, some commit checks broke.

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worker/draft.go, line 242 at r2 (raw file):

		cfg: &raft.Config{
			ID: id,
			// 200 ms if we call Tick() every 20 ms.

The comments can fit in the same line as the code.

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worker/draft.go, line 244 at r2 (raw file):

			// 200 ms if we call Tick() every 20 ms.
			ElectionTick: 10,
			// 20 ms if we call Tick() every 20 ms.

and here.

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Comments from Reviewable

[srh]

In master as of f1ac05e