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
(idle comment)  I wonder if there's a good, open source timing/micro-benchmarking project we should consider (rather than rolling our own timing code)?