API to delete multiple ledgers at once

**FEATURE REQUEST**

1. Please describe the feature you are requesting.
Add the ability to drop a few ledgers in a single atomic operation.

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have).
should-have

3. Provide any additional detail on your proposed use case for this feature.

We could leverage ZooKeeper "multi" operation
https://zookeeper.apache.org/doc/r3.5.5/api/org/apache/zookeeper/ZooKeeper.html#multi(java.lang.Iterable).

Sometimes it happens that you need to delete multiple ledgers at once, and deleting only a single ledger could lead to an inconsistent state of the system (partial deletion of data).
Actually in case of a failure you need to implement garbage collection like mechanisms that scan for orphan ledgers and delete them.

Start by locating the existing single-ledger deletion API and its tests, then read the linked ZooKeeper multi-operation documentation. The work is done when multiple ledger deletions are exposed through one atomic operation and failure behavior is covered so partial deletion does not leave inconsistent state.

Written by the indexing model from the issue text.