Replace non-public MapCounter with DataWave utility

[SethSmucker]

Summary

Replace non-public org.apache.accumulo.core.util.MapCounter with a DataWave-specific utility class.

Non-Public Class

• org.apache.accumulo.core.util.MapCounter

Solution

Create a simple datawave.util.MapCounter class that provides the same API for counting occurrences of keys.

Files Affected

• warehouse/accumulo-extensions/src/test/java/datawave/ingest/table/balancer/ShardedTableTabletBalancerTest.java
• warehouse/accumulo-extensions/src/test/java/datawave/util/MapCounter.java (new)

Part of #2443