GEOMESA-583 Adding customized encoding/decoding for index entries

* Replacing IndexEntry encode/decode with IndexValueEncoder
* Adding ability to specify attributes to encode in st index value to SimpleFeatureTypes, SftBuilder
* Selecting index iterator in the case of a transform covered by the custom index value
* Improving method profiling
i changed the encode methods to take/return a byte array instead of a Value. The .get returns the Value's byte array.