Add Tablet.SetTimestampAt with rowIndex validation

[PDGGK]

Problem

Tablet.SetTimestamp writes to t.timestamps[rowIndex] without validating
rowIndex, so an out-of-range row panics with an index-out-of-range error
instead of returning one. For example, with a tablet created via
NewTablet(..., 1) (only row index 0 is valid), SetTimestamp(ts, 1) panics.

Fix

As @HTHou noted, adding an error return to SetTimestamp would break source
compatibility for consumers relying on its signature (interface satisfaction,
method values), and this module already has v2 releases. So SetTimestamp is
left unchanged, and a checked alternative is added:

func (t *Tablet) SetTimestampAt(timestamp int64, rowIndex int) error {
	if rowIndex < 0 || rowIndex >= t.maxRowNumber {
		return fmt.Errorf("illegal argument rowIndex %d", rowIndex)
	}
	t.timestamps[rowIndex] = timestamp
	return nil
}

SetTimestampAt mirrors the bounds handling of SetValueAt / GetValueAt,
returning an illegal argument rowIndex error for a negative or out-of-range
row. The PR is purely additive; SetTimestamp can be deprecated in
documentation later if callers should migrate to the checked API.

Tests

Added TestTablet_SetTimestampAtRowIndexBounds: a valid row index returns no
error; out-of-range indices (1 on a one-row tablet, and -1) return an error
instead of panicking. go build ./... and go test ./client/ pass.

Follow-up to #168.

[HTHou]

Could we preserve the existing SetTimestamp(timestamp int64, rowIndex int) signature and introduce a checked API such as SetTimestampAt(timestamp int64, rowIndex int) error instead?

Adding an error result to the existing exported method is not fully source-compatible. Statement-form calls still compile, but consumers using an interface or method value will break, for example:

type TimestampSetter interface {
    SetTimestamp(int64, int)
}

var _ TimestampSetter = (*client.Tablet)(nil)
var setter func(int64, int) = tablet.SetTimestamp

Since this module already has v2 releases, keeping SetTimestamp unchanged would preserve compatibility, while SetTimestampAt can perform the new bounds validation and return illegal argument rowIndex errors. The existing method could be deprecated in documentation later if callers should migrate to the checked API.

[PDGGK]

Thanks @HTHou — good point on the interface/method-value compatibility, and you're right that a v2 public API shouldn't change SetTimestamp's signature. Adopted: SetTimestamp is left exactly as-is, and I added SetTimestampAt(timestamp int64, rowIndex int) error doing the bounds validation (returning illegal argument rowIndex, consistent with SetValueAt/GetValueAt). The PR is now purely additive. Happy to add a doc note steering callers to the checked API if you'd like.