Make S3 Capability More Configurable

[jamesmcclain]

The val s3Client lines have been changed to def s3Client in both S3ValueReader and S3AttributeStore. Examples of how to override them have been included, as well.

Connects #2304

[echeipesh]

+💯 on adding the documentation example

[pomadchin]

Does it break binary compatibility and do we care about it too much? (well nobody called changed things in a client code i believe) 💃

[echeipesh]

It shouldn't but loading it up to inspect it using the mima plugin atm

[pomadchin]

Looks like it's clean.

s3 >  mimaReportBinaryIssues
[info] Resolving com.amazonaws#jmespath-java;1.11.92 ...
[info] geotrellis-s3: found 8 potential binary incompatibilities while checking against org.locationtech.geotrellis:geotrellis-s3_2.11:1.1.0 
[error]  * method apply(org.apache.spark.rdd.RDD,scala.Function2,geotrellis.spark.io.s3.S3GeoTiffRDD#Options,geotrellis.spark.io.RasterReader)org.apache.spark.rdd.RDD in object geotrellis.spark.io.s3.S3GeoTiffRDD does not have a correspondent in current version
[error]    filter with: ProblemFilters.exclude[DirectMissingMethodProblem]("geotrellis.spark.io.s3.S3GeoTiffRDD.apply")
[error]  * synthetic method geotrellis$spark$io$s3$S3RDDReader$_setter_$DefaultThreadCount_=(Int)Unit in trait geotrellis.spark.io.s3.S3RDDReader is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("geotrellis.spark.io.s3.S3RDDReader.geotrellis$spark$io$s3$S3RDDReader$_setter_$DefaultThreadCount_=")
[error]  * method DefaultThreadCount()Int in trait geotrellis.spark.io.s3.S3RDDReader is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("geotrellis.spark.io.s3.S3RDDReader.DefaultThreadCount")
[error]  * synthetic method update$default$7()Int in trait geotrellis.spark.io.s3.S3RDDWriter is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("geotrellis.spark.io.s3.S3RDDWriter.update$default$7")
[error]  * method update(org.apache.spark.rdd.RDD,java.lang.String,scala.Function1,scala.Option,scala.Option,scala.Function1,Int,geotrellis.spark.io.avro.AvroRecordCodec,scala.reflect.ClassTag,geotrellis.spark.io.avro.AvroRecordCodec,scala.reflect.ClassTag)Unit in trait geotrellis.spark.io.s3.S3RDDWriter is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("geotrellis.spark.io.s3.S3RDDWriter.update")
[error]  * synthetic method update$default$6()scala.Function1 in trait geotrellis.spark.io.s3.S3RDDWriter is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("geotrellis.spark.io.s3.S3RDDWriter.update$default$6")
[error]  * synthetic method geotrellis$spark$io$s3$S3RDDWriter$_setter_$DefaultThreadCount_=(Int)Unit in trait geotrellis.spark.io.s3.S3RDDWriter is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("geotrellis.spark.io.s3.S3RDDWriter.geotrellis$spark$io$s3$S3RDDWriter$_setter_$DefaultThreadCount_=")
[error]  * method DefaultThreadCount()Int in trait geotrellis.spark.io.s3.S3RDDWriter is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("geotrellis.spark.io.s3.S3RDDWriter.DefaultThreadCount")
[trace] Stack trace suppressed: run last s3/*:mimaReportBinaryIssues for the full output.
[error] (s3/*:mimaReportBinaryIssues) geotrellis-s3: Binary compatibility check failed!
[error] Total time: 7 s, completed Oct 10, 2017 6:36:56 PM