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
the 'stidx' param indicates that it gets encoded in the index entry value, not that it's part of the key. Might be slightly confusing... I couldn't come up with a good, short parameter that described it very well.
I added it to the geometry to keep in line with always including 'index=true'.