Process seeding of split fate operations in batches

Updates the Seeder in the Manager that handles seeding split fate ops to
use a single thread and to submit multiple outstanding operations to
be seeded together instead of individually in order to improve
performance. The user fate store will now track outstanding fate
operations and return a future for each pending operation that will be
completed when the batch is submitted.

This closes apache#5160

Author: cshannon

core/src/main/java/org/apache/accumulo/core/conf/Property.java

@@ -461,7 +461,7 @@ public enum Property {
           + "indefinitely. Default is 0 to block indefinitely. Only valid when tserver available "
           + "threshold is set greater than 0.",
       "1.10.0"),
-  // TODO: Mark this as deprecated or delete?
+  @Deprecated(since = "4.0.0")
   MANAGER_SPLIT_WORKER_THREADS("manager.split.seed.threadpool.size", "8", PropertyType.COUNT,
       "The number of threads used to seed fate split task, the actual split work is done by fate"
           + " threads.",

server/manager/src/main/java/org/apache/accumulo/manager/split/Splitter.java

@@ -74,6 +74,8 @@ public void run() {
           final Map<Text,KeyExtent> userSplits = new HashMap<>();
           final Map<Text,KeyExtent> metaSplits = new HashMap<>();
 
+          // Go through all the queued up splits and partition
+          // into the different store types to be submitted.
           queuedTablets.forEach((metaRow, extent) -> {
             switch (FateInstanceType.fromTableId((extent.tableId()))) {
               case USER:
@@ -87,6 +89,9 @@ public void run() {
             }
           });
 
+          // see the user and then meta splits
+          // The meta plits (zk) will be processed one at a time but there will not be
+          // many of those splits. The user splits are processed as a batch.
           seedSplits(FateInstanceType.USER, userSplits);
           seedSplits(FateInstanceType.META, metaSplits);
         }
@@ -212,9 +217,6 @@ public Splitter(Manager manager) {
     this.manager = manager;
     ServerContext context = manager.getContext();
 
-    // TODO: Mark this as deprecated or delete?
-    // int numThreads = context.getConfiguration().getCount(Property.MANAGER_SPLIT_WORKER_THREADS);
-
     this.splitExecutor = context.threadPools().getPoolBuilder("split_seeder").numCoreThreads(1)
         .numMaxThreads(1).withTimeOut(0L, TimeUnit.MILLISECONDS).enableThreadPoolMetrics().build();