GEOMESA-126 Improve GeoServer WMS GetCapabilities handling for time enab...

...led layers

For time-enabled layers, GeoServer calculates the time extent by looking at all the features in a FeatureStore.
For large datasets, this is incredibly slow.  This pr helps rectify this issue by

* Calculating and updating a time interval while writing features to GeoMesa.
* AccumuloFeatureCollections retrieve stored time extent data in response to Min/MaxVisitors.
  If no such metadata is present, the interval returned is 1970 until now.
* Updating the Caching functionality to return a new CachingAccumuloFeatureCollection.
  Such CachingAFCs provide access to the AFC class and store the results of 'features' in a private lazy val.

NB: In order for this to work properly, there are two GeoServer related caveats:
1.  The SimpleFeatureType's name must match the layer name in GeoServer.
If not, then GeoServer will wrap the AFC in a retyping collection.
2.  Given the current state of CRS handling, the layer must use SRS Handling 'Keep Native'.
If not, GeoServer will wrap the AFC in a collection to force the declared CRS.

Either of these wrappings mess prevent GeoMesa from intercepting the visitors as intended and produce full table scans.
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