GEOMESA-287,GEOMESA-303 Handle transformations in command line export Investigate export behavior when exporting with -a id

To test this PR, sample commands can be found at https://geomesa.atlassian.net/browse/GEOMESA-287.
GEOMESA-419 Refactor metadata out of AccumuloDataStore

* Created GeoMesaMetadata trait and Accumulo implementation
* Moved metadata methods into it
* Used concept of key/value store for metadata as String,String instead of Text,String