Replace deprecated AccumulatorParam with AccumulatorV2

[heuermh]

No description provided.

[heuermh]

@akmorrow13 This is to support Spark 3, which removes AccumulatorParam.

Note also that the utils-metrics package may have to be dropped for Spark 3, as it uses too many Spark internals to safely straddle the Spark 2/3 divide.

[coveralls]

Coverage increased (+0.2%) to 54.393% when pulling 8aa59e0 on heuermh:accumulator-v2 into 79cadd2 on bigdatagenomics:master.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/bdg-utils-prb/155/
Test PASSed.

[akmorrow13]

Thanks for doing this @heuermh! LGTM

[heuermh]

Thank you, @akmorrow13