Support the temporary lock up of a space to ensure that CLEAR SPACE and actually clear the space

[randomJoe211]

Is your feature request related to a problem? Please describe.
It seems like when trying to run CLEAR SPACE we have no good way to stop writing. Users may have trouble clearing a graph space while somebody else is still writing data into the space, and since they need to run CLEAR SPACE, the Graph service cannot be stopped to block writing.

Describe the solution you'd like
Shall we add a query to stop clients from writing into a certain space (lock up a space) for a period of time?

[sworduo]

Snapshot::instance(kvstore_, client_)->blockingWrites(SignType::BLOCK_ON) maybe helpful.

[zhaohaifei]

nebula-ent can be set to read-only before clear. But nebula cannot.

[randomJoe211]

Thank you both for the reply! Good to know that we have more than one way to do this. 👍