Reduce HostRegexTableLoadBalancer RPCs for OOB checks (#6446)

The StatusThread.updateStatus() method in the Manager calls
gatherTableInformation to get information about all of the
TabletServers in the cluster, then calls balanceTablets().

balanceTablets() creates a partitioned view of the status
information and uses that when creating the BalanceParams
to pass to table balancer's balance().

The HostRegexTableLoadBalancer.balance method will periodically
check for tablets that are hosted outside of the defined
regex. Prior to this change it was making an RPC call to
every tablet server outside of the defined regex.

This change uses the gathered tablet server information
such that the RPC call is made to the tablet server if
the information indicates that the tablet server is
hosting tablets for the table. The RPC is necessary
to get the extent information to create the migrations,
but it's only necessary when the tablet server is
hosting tablets for the table.

Author: dlmarion

core/src/main/java/org/apache/accumulo/core/spi/balancer/HostRegexTableLoadBalancer.java

@@ -421,6 +421,14 @@ public long balance(BalanceParameters params) {
                   + " it may have been deleted or renamed.", table);
               continue;
             }
+            // If current information in the Manager indicates that the tserver
+            // is not hosting tablets for the table, then move on to the next
+            // tserver
+            if (e.getValue().getTableMap() != null
+                && !e.getValue().getTableMap().containsKey(tid.canonical())) {
+              LOG.debug("TServer {} is not hosting any tablets for table {}", e.getKey(), table);
+              continue;
+            }
             try {
               List<TabletStatistics> outOfBoundsTablets = getOnlineTabletsForTable(e.getKey(), tid);
               if (outOfBoundsTablets == null) {