fix: deadlock when stop keepalive bg task

[ShiKaiWi]

Related Issues

Closes #

Detailed Changes

When shard lock is expired, the on_expired callback will be executed in the shard lock lease keepalive background task, and the callback will stop the background task and wait for its finish, leading to a deadlock. This change set fixes it by avoid waiting for the background task to finish.

Test Plan

Test it by these steps:

• Start ceresmeta and ceresdb;
• Wait all the shards to open on the ceresdb;
• Stop ceresmeta to let the shard lock lease be expired;
• Observe whether the shard lock can be revoked successfully.

[jiacai2050]

LGTM