allow lock timeout for a slice of a sharded task to be configured via `MapSettings`

[eschultink]

Basic problem is that you can configure millisPerSlice via MapSettings. When a MapJob runs, it continues to execute the slice until it's been executing for >= that many milliseconds (see example implementation in MapShardTask).

While this shard task is executing, the share is "locked". This lock is considered to be held for a timeout that is millisPerSlice * 1.1 (MapSettings:286). If that lock has not been released and that much time has passed, ShardJobRunner will deem the slice as failed (see ShardJobRunner.handleLockHeld()).

This PR makes that 1.1 value configurable via MapSettings (or MapReduceSettings). I've called this sliceTimeoutRatio, but certainly open to suggestions. This PR also replaces a few other magic numbers with the appropriate constants.

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[eschultink]

I signed it!

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CLAs look good, thanks!

[eschultink]

@aozarov thanks Arie - removed that.