Look into using a Striped lock for bulk load zookeeper checks.

Bulk load RPCs check to see if the transaction is still alive in ZooKeeper before executing. While doing this check a tablet server wide [lock is held](https://github.com/apache/accumulo/blob/11e26d481b7885e46ce439e1345f0bf9bdd5dfb2/server/base/src/main/java/org/apache/accumulo/server/zookeeper/TransactionWatcher.java#L126). Therefore concurrent bulk loads executing different transactions may have to wait on each other. This could be avoided by using a [Striped] lock.

It's possible that allowing more concurrency may not be a good thing in that it could place more load on zookeeper. Could possibly make the stripe size small to allow a small amount of concurrency.

[Striped]: https://static.javadoc.io/com.google.guava/guava/27.0.1-jre/com/google/common/util/concurrent/Striped.html

Read server/base/src/main/java/org/apache/accumulo/server/zookeeper/TransactionWatcher.java at the linked lock location, then review the closed pull request #1710. Check how bulk-load transaction checks are synchronized and whether striped locking changes concurrency without imposing unacceptable ZooKeeper load; done means the approach is evaluated and its behavior is validated.

Written by the indexing model from the issue text.