Add SettableGauge interface and an implementation

[xiaochuanyu]

This PR was re-made from #1599 to change source (and target branch).

This PR attempts to address #1126.

Example usage:

MetricRegistry registry = new MetricRegistry();
SettableGauge<Integer> settable = registry.register("example1", new SimpleSettableGauge<>(0));
settable.setValue(123);

[xiaochuanyu]

@joschi I've made this new PR to target release/4.2.x per your comment from the original PR.

@the-thing I've addressed the comments from the original PR in 9fe9105 here.

[the-thing]

LGTM. The name of the default implementation might not suit some people, because the existing naming convention favours "Default" prefix such as com.codahale.metrics.jdbi3.strategies.DefaultNameStrategy or com.codahale.metrics.jmx.DefaultObjectNameFactory.

[joschi]

Merged in f4d84b6.