Cancel background merges early in DROP DATABASE for ordinary shared merge tree

[tiandiwonder]

In InterpreterDropQuery::executeToDatabaseImpl(), currently flushAndPrepareForShutdown() is not override, so we depends StorageMergeTree::shutdown() to cancel background merges in executeToTableImpl() in sequence

So we override StorageMergeTree::flushAndPrepareForShutdown() to cancel background merges early in prepare stage in parallel.

            /// List the tables, then call flushAndPrepareForShutdown() on them in parallel, then call
            /// executeToTableImpl on them in sequence.

Note: we already override StorageReplicatedMergeTree::flushAndPrepareForShutdown() for replicated merge tree.

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes into CHANGELOG.md):

Cancel background merges early in DROP DATABASE for ordinary shared merge tree.

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

[clickhouse-gh]

Workflow [PR], commit [12d7923]

Summary: ✅